Nitride semiconductor light-emitting element

ABSTRACT

A nitride semiconductor light-emitting element includes at least an n-type nitride semiconductor layer, a light-emitting layer, and a p-type nitride semiconductor layer. A multilayer body is provided between the n-type nitride semiconductor layer and the light-emitting layer, having at least one stack of first and second semiconductor layers. The second semiconductor layer has a greater band-gap energy than the first semiconductor layer. The first and second semiconductor layers each have a thickness of more than 10 nm and 30 nm or less. In applications in which luminous efficiency at room temperature is a high priority, the first semiconductor layer has a thickness of more than 10 nm and 30 nm or less, the second semiconductor layer has a thickness of more than 10 nm and 40 nm or less, and the light-emitting layer has V-shaped recesses in cross-sectional view.

TECHNICAL FIELD

The present invention relates to a nitride semiconductor light-emittingelement.

BACKGROUND ART

Nitrogen-containing Group III-V compound semiconductors (Group IIInitride semiconductors) have a band-gap energy that corresponds to theenergy of light of infrared to ultraviolet wavelengths. This makes GroupIII nitride semiconductors useful materials for light-emitting elementsthat emit light of infrared to ultraviolet wavelengths and forlight-receiving elements that receive light of infrared to ultravioletwavelengths.

Group III nitride semiconductors are composed of atoms bonded togetherby strong atomic forces and have a high dielectric breakdown voltage anda large saturated electron velocity. These make Group III nitridesemiconductors useful materials for electronic devices such ashigh-temperature-resistant and high-power radiofrequency transistors,too. Practically harmless to the environment, furthermore, Group IIInitride semiconductors have been receiving attention as easy-to-handlematerials.

A nitride semiconductor light-emitting element in which such a Group IIInitride semiconductor is used typically has a quantum-welllight-emitting layer. When voltage is applied to the nitridesemiconductor light-emitting element, electrons and holes are recombinedin a well layer as a component of the light-emitting layer and generatelight. The light-emitting layer may have the Single Quantum Well (SQW)structure, or may alternatively have the Multiple Quantum Well (MQW)structure, in which well layers are stacked alternately with barrierlayers.

The well layers in the light-emitting layer are usually InGaN layers,and the barrier layers are usually GaN layers. The resulting device is,for example, a blue LED (Light Emitting Diode) having a peak emissionwavelength of approximately 450 nm, and this blue LED can be combinedwith a phosphor to form a white LED. When the barrier layers are AlGaNlayers, the increased difference in band-gap energy between the barrierand well layers will lead to enhanced luminous efficiency. AlGaN,however, is difficult to grow into crystals with good quality comparedwith GaN.

Typical N-type nitride semiconductor layers used in nitridesemiconductor light-emitting elements are GaN and InGaN layers.

For example, the nitride semiconductor light-emitting element describedin Japanese Unexamined Patent Application Publication No. 11-214746(PTL 1) has, between a substrate and a light-emitting layer, a firstnitride semiconductor layer having an n-type impurity of 1×10¹⁷ cm⁻³ orless, a second nitride semiconductor layer having an n-type impurity of3×10¹⁸ cm⁻³ or less, and a third nitride semiconductor layer having ann-type impurity of 1×10¹⁷ cm⁻³ or less, with the first one closest tothe substrate. According to PTL 1, the low n-type impurityconcentrations of the first and third layers make these layers highlycrystalline underlying layers, and the good crystallinity of the firstlayer helps the second layer, which has a higher n-type impurityconcentration, grow with good crystallinity on the first layer.

Japanese Unexamined Patent Application Publication No. 11-330554 (PTL 2)describes a nitride semiconductor light-emitting element that has alight-emitting layer between an n-type nitride semiconductor layer and ap-type nitride semiconductor layer. In this nitride semiconductorlight-emitting element, the n-type nitride semiconductor layer is ann-type multilayer film layer that is a stack of an In-containing firstnitride semiconductor layer and a second nitride semiconductor layerwhose composition is different from that of the first nitridesemiconductor layer. At least one of the first and second nitridesemiconductor layers has a thickness of 100 Angstroms or less. Accordingto PTL 2, high crystallinity of the light-emitting layer, gained as aresult of the superlattice structure of the n-type multilayer film layerin particular, improves the efficiency of the nitride semiconductorlight-emitting element.

The nitride semiconductor device described in Japanese Unexamined PatentApplication Publication No. 10-126006 has a first nitride semiconductorlayer on and in contact with at least one side of a light-emittinglayer. The first nitride semiconductor layer has a greater band-gapenergy than the light-emitting layer, and second and third nitridesemiconductor layers are provided on the first nitride semiconductorlayer. The second nitride semiconductor layer has a smaller band-gapenergy than the first nitride semiconductor layer, and the third nitridesemiconductor layer has a greater band-gap energy than the secondnitride semiconductor layer. According to PTL 3, the invention providesa nitride semiconductor device with high luminous efficiency.

Another disclosed structure is aimed at improving optical power andreducing leakage current and includes V-pits created in an upper portionof an n-type nitride semiconductor layer. The V-pits are carried over toan active layer and closed by a p-type nitride semiconductor layer. Theimportance is on a structure of the n-type nitride semiconductor layerand a formation method that provide desirable V-pits.

Japanese Patent No. 3904709 (PTL 4) discloses a structure that includesan “n-type In_(0.1)Ga_(0.9)N/In_(0.02)Ga_(0.98)N multiple quantum welladjacent layer (Si-doped, 5×10¹⁷ cm⁻³; well width, 2 nm; barrier width,4 nm; 20 layers),” an “In_(0.2)Ga_(0.8)N/In_(0.05)Ga_(0.95)N multiplequantum well active layer (undoped; well width, 2 nm; barrier width, 4nm; 10 layers)” thereon, and a “p-type GaN adjacent layer (Mg-doped,5×10¹⁷ cm⁻³; 0.1 μm) thereon. The multiple quantum well adjacent layerhas pits. The pits are carried over to the multiple quantum well activelayer above and closed by the p-type GaN adjacent layer. The multiplequantum well adjacent layer has a structure now commonly referred to asthe superlattice structure after its configuration.

Japanese Patent No. 3612985 (PTL 5) discloses that forming a 0.5-μmthick silicon-doped GaN layer (electron concentration, 1×10¹⁸/cm³) as a“strain relief layer” under an active layer at relatively lowtemperatures generates many V-pits, with some in the active layer.According to PTL 5, this significantly improves the photoluminescencecharacteristics of the active layer.

Japanese Patent No. 5415756 (PTL 6) and Japanese Patent No. 5603366 (PTL7) disclose structures that have a superlattice layer referred to as apit opening layer under an active region (active layer). Quantum welllayers and a hole injection layer extend into pits originating fromthreading dislocations, and the pits are closed by a p-type contactlayer. According to PTL 6 and 7, this improves luminous and wall-plugefficiency.

CITATION LIST Patent Literature

PTL 1: Japanese Unexamined Patent Application Publication No. 11-214746

PTL 2: Japanese Unexamined Patent Application Publication No. 11-330554

PTL 3: Japanese Unexamined Patent Application Publication No. 10-126006

PTL 4: Japanese Patent No. 3904709

PTL 5: Japanese Patent No. 3612985

PTL 6: Japanese Patent No. 5415756

PTL 7: Japanese Patent No. 5603366

SUMMARY OF INVENTION Technical Problem

A nitride semiconductor light-emitting element typically has a(strained-layer) superlattice structure (the (strained-layer)superlattice structure is formed of a nitride semiconductor) composed ofperiodically stacked layers having thicknesses of 10 nm (100 Å) or less(e.g., approximately 1 to 6 nm) under a light-emitting layer. Thisreportedly enables effective relaxation of the strain put on thelight-emitting layer and thus ensures good light-emittingcharacteristics.

For further improvements in light-emitting characteristics, however, itis also important to reduce the density of threading dislocations in thelight-emitting layer besides strain relaxation. In particular,improvements in the thermal characteristics of light-emitting elementscannot be achieved without reducing the density of threadingdislocations. Thermal characteristics of a light-emitting element asmentioned herein refers to the proportion of luminous efficiency at ahigh temperature (e.g., 80° C.) to that at room temperature. In general,the thermal characteristics of a light-emitting element decreases withelevating operating temperature of the light-emitting element. From thepractical perspective, light-emitting elements need to have high thermalcharacteristics.

According to findings from the inventors' recent research, however, theabove configuration in which a (strained-layer) superlattice structureis disposed under a light-emitting layer is of limited effectiveness inreducing the density of threading dislocations.

Meanwhile, there is increasing evidence that in applications in whichluminous efficiency at room temperature is a high priority, such as LEDsfor use as backlighting in mobile liquid crystal displays, it isimportant to form an active layer with good crystallinity and, inaddition to this, create V-pits in the active layer. Two models havebeen proposed to explain the role of V-pits which has not been fullyunderstood: an increased luminous efficiency resulting from directinjection of holes from the V-pits into the quantum wells as a componentof the light-emitting layer; and an increased luminous efficiency as aresult of layers by which the V-pits are closed serving as barrierlayers that prevent carriers in the quantum well layers from being lost.Those LEDs that have a V-pitted light-emitting layer also generally havea superlattice structure as an underlying layer for the light-emittinglayer.

Made in light of the foregoing, the present invention is intended tofurther improve the light-emitting characteristics of nitridesemiconductor light-emitting elements.

Solution to Problem

The inventors found that the density of threading dislocations in alight-emitting layer is more effectively reduced by placing a multilayerbody of n-type nitride semiconductor layers under the light-emittinglayer. A multilayer body of n-type nitride semiconductor layers asmentioned herein refers to a structure composed of periodically stackedlayers with different band-gap energies and relatively large thicknesses(e.g., layers having a thickness of more than 10 nm and 30 nm or less).In other words, the inventors found that when each of the n-type nitridesemiconductor layers constituting the multilayer body has a thickness ofmore than 10 nm and 30 nm or less, the light-emitting layer has areduced density of threading dislocations as a result of threadingdislocations being deflected at the interfaces between layers withdifferent band-gap energies.

The inventors also found that those LEDs for applications in whichluminous efficiency at room temperature is a high priority can have theabove multilayer body, rather than the underlying structure, as anunderlying structure with high planarity of its crystal growth surface,and that combining this multilayer body with a light-emitting layerhaving V-shaped recesses (V-pits) further improves luminous efficiency.

A nitride semiconductor light-emitting element according to the presentinvention includes at least an n-type nitride semiconductor layer, alight-emitting layer, and a p-type nitride semiconductor layer. Amultilayer body is provided between the n-type nitride semiconductorlayer and the light-emitting layer, and the multilayer body has at leastone stack of first and second semiconductor layers. The secondsemiconductor layer has a greater band-gap energy than the firstsemiconductor layer. Each of the first and second semiconductor layershas a thickness of more than 10 nm and 30 nm or less.

A nitride semiconductor light-emitting element according to the presentinvention includes, in an LED for applications in which luminousefficiency at room temperature is a high priority, at least an n-typesemiconductor layer, a light-emitting layer, and a p-type semiconductorlayer. A multilayer body is provided between the n-type nitridesemiconductor layer and the light-emitting layer, and the multilayerbody has at least one stack of first and second semiconductor layers.The second semiconductor layer has a greater band-gap energy than thefirst semiconductor layer. The first semiconductor layer has a thicknessof more than 10 nm and 30 nm or less. The second semiconductor layer hasa thickness of more than 10 nm and 40 nm or less. The light-emittinglayer has a plurality of V-shaped recesses (V-pits) in cross-sectionalview.

Preferably, the first semiconductor layer is anAl_(x1)In_(y1)Ga_(1-x1-y1)N (0≤x1<1 and 0<y1≤1) layer, and the secondsemiconductor layer is an Al_(x2)In_(y2)Ga_(1-x2-y2)N (0≤x2<1 and0≤y2<1) layer.

Preferably, each of the first and second semiconductor layers has ann-type impurity concentration of 3×10¹⁸ cm⁻³ or more and less than1.1×10¹⁹ cm⁻³. More preferably, the first and second semiconductorlayers have equal n-type impurity concentrations.

Preferably, the first and second semiconductor layers have equalthicknesses. Preferably, the multilayer body has three to seven stacksof the first and second semiconductor layers. More preferably, thelight-emitting layer lies in contact with the multilayer body. In thiscase, the second semiconductor layer closest to the light-emitting layeris in contact with the light-emitting layer.

Preferably, an n-type buffer layer (the second n-type buffer layerdiscussed hereinafter) is provided between the multilayer body and thelight-emitting layer. Preferably, the n-type buffer layer is anAl_(x3)In_(y3)Ga_(1-x3-y3)N (0≤x3<1 and 0≤y3<1) layer that contains ann-type impurity and lies in contact with the light-emitting layer.

The band-gap energy of the n-type buffer layer may be equal to orgreater than that of the second semiconductor layer, equal to or lessthan that of the first semiconductor layer, or smaller than that of thesecond semiconductor layer and greater than that of the firstsemiconductor layer.

Preferably, the n-type buffer layer has a thickness of 30 nm or less.

Preferably, an n-type buffer layer (the first n-type buffer layerdiscussed hereinafter) is provided between the n-type nitridesemiconductor layer and the multilayer body. Preferably, the n-typebuffer layer between the n-type nitride semiconductor layer and themultilayer body is an Al_(s4)In_(t4)Ga_(1-s4-t4)N (0≤s4<1 and 0≤t4<1)layer that contains an n-type impurity and lies in contact with themultilayer body.

Preferably, the n-type buffer layer between the n-type nitridesemiconductor layer and the multilayer body has a band-gap energy equalto that of the second semiconductor layer. Preferably, the n-type bufferlayer between the n-type semiconductor layer and the multilayer body hasan n-type impurity concentration equal to at least one of the n-typeimpurity concentrations of the first and second semiconductor layers.Preferably, the n-type buffer layer between the n-type nitridesemiconductor layer and the multilayer body has a thickness of 50 nm orless.

The light-emitting layer is preferably an undoped layer, more preferablyhaving the single quantum well structure or a multiple quantum wellstructure in which well layers are stacked alternately withAl_(f)In_(g)Ga_(1-f-g)N (0≤f≤0.01 and 0≤g≤0.01) barrier layers.

Preferably, the light-emitting layer has V-shaped recesses (V-pits) incross-sectional view that reach the multilayer body at the bottom of theV-shape thereof.

Preferably, the V-shaped recesses (V-pits) are present as a large numberof scattered cavities in plan view of the top portion of thelight-emitting layer with the plane surface density of the V-shapedrecesses (V-pits) being 1×10⁸/cm² or more.

Advantageous Effects of Invention

The present invention further improves the light-emittingcharacteristics of nitride semiconductor light-emitting elements.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 (a) is a cross-section of a nitride semiconductor light-emittingelement according to an embodiment of the present invention. FIG. 1 (b)is an enlarged view of features of a nitride semiconductorlight-emitting element according to an embodiment of the presentinvention.

FIG. 2 is a plan view of a nitride semiconductor light-emitting elementaccording to an embodiment of the present invention.

FIG. 3 (a) is a graph of the relative excitation intensity of PL (PhotoLuminescence) emitted by a nitride semiconductor light-emitting elementaccording to an embodiment of the present invention versus Si level inits first and second semiconductor layers. FIG. 3 (b) is a graph of thethermal characteristics of the luminous intensity of a nitridesemiconductor light-emitting element according to an embodiment of thepresent invention versus Si level in its first and second semiconductorlayers.

FIG. 4 is an AFM (Atomic Force Microscopy) image of a nitridesemiconductor light-emitting element according to an embodiment of thepresent invention observed immediately before the formation of itslight-emitting layer.

FIG. 5 is an energy band diagram that schematically illustrates the bandstructure of the nitride semiconductor light-emitting element accordingto Example 1.

FIG. 6 is an energy band diagram that schematically illustrates the bandstructure of the nitride semiconductor light-emitting element accordingto Example 2.

FIG. 7 is an energy band diagram that schematically illustrates the bandstructure of the nitride semiconductor light-emitting element accordingto Example 3.

FIG. 8 is an energy band diagram that schematically illustrates the bandstructure of the nitride semiconductor light-emitting element accordingto Example 4.

FIG. 9 is an energy band diagram that schematically illustrates the bandstructure of the nitride semiconductor light-emitting element accordingto Example 5.

FIG. 10 is an AFM (Atomic Force Microscopy) image of a nitridesemiconductor light-emitting element according to another embodiment ofthe present invention, developed with a focus on room-temperaturecharacteristics, observed immediately before the formation of itslight-emitting layer.

FIG. 11 is a cross-section of a room temperaturecharacteristics-oriented nitride semiconductor light-emitting elementaccording to this embodiment of the present invention.

FIG. 12 is a plan view of a nitride semiconductor light-emitting elementaccording to this embodiment of the present invention.

FIG. 13 is an energy band diagram that schematically illustrates theband structure of the nitride semiconductor light-emitting elementaccording to Example 10.

FIG. 14 is an energy band diagram that schematically illustrates theband structure of the nitride semiconductor light-emitting elementaccording to Example 11.

FIG. 15 is an energy band diagram that schematically illustrates theband structure of the nitride semiconductor light-emitting elementaccording to Example 12.

DESCRIPTION OF EMBODIMENTS

The following describes nitride semiconductor light-emitting elementsaccording to the present invention with reference to drawings. In thedrawings of the present invention, the same reference numerals refer tothe same or corresponding parts. To make the drawings clear and simple,the proportions of dimensions such as lengths, widths, thicknesses, anddepths are not to scale and do not represent actual proportions.

In the following, portions on the lower side direction of FIG. 1 (a) maybe described using words such as “lower,” “below,” “under,” “beneath,”and “bottom,” and those on the upper side direction of FIG. 1 (a) may bedescribed using words such as “upper,” “above,” “on,” and “top.” Theseexpressions are for the sake of convenience and different from the terms“upper” and “lower” etc. according to the direction of gravity.

A “well layer 14W” refers to a layer interposed between barrier layers(e.g., see FIG. 5). A well layer not interposed between barrier layersis referred to as “the initial well layer 14WI” or “the final well layer14WF” (e.g., see FIG. 5); well layers are termed differently accordingto whether or not they are interposed between barrier layers. “Theinitial well layer 14WI” is on the n-type nitride semiconductor layerside, and “the final well layer 14F” is on the p-type nitridesemiconductor layer side.

In the following, two different concentrations are used, “impurityconcentration” and “carrier concentration,” the latter being theconcentration of electrons resulting from doping with an n-type impurityor that of holes resulting from doping with a p-type impurity. Therelationship between a “n-type impurity concentration” and a “carrierconcentration” is discussed hereinafter.

A “carrier gas” is a gas other than Group III, Group V, and impurityraw-material gases. The atoms constituting a carrier gas are notincorporated into any component such as films.

An “n-type nitride semiconductor layer” may include alow-carrier-concentration n-type or undoped layer whose thickness issuch that electrons are practically free to pass through the layer. An“p-type nitride semiconductor layer” may include alow-carrier-concentration p-type or undoped layer whose thickness issuch that holes are practically free to pass through the layer. The term“practically free to pass through the layer” means that the nitridesemiconductor light-emitting element has an operating voltage suitablefor practical use.

<Structure of the Nitride Semiconductor Light-Emitting Element>

FIG. 1 (a) is a cross-section of a nitride semiconductor light-emittingelement according to an embodiment of the present invention. FIG. 1 (b)is an enlarged view of features of the nitride semiconductorlight-emitting element illustrated in FIG. 1 (a). In FIG. 1 (a), regionIA illustrates a cross-sectional structure viewed along line IA-IA inFIG. 2, and region IB illustrates a cross-sectional structure viewedalong line IB-IB in FIG. 2.

The nitride semiconductor light-emitting element 1 in FIG. 1 includes asubstrate 3, a buffer layer 5, an underlying layer 7, an n-type contactlayer (n-type nitride semiconductor layer) 8, a first n-type bufferlayer (n-type buffer layer between an n-type nitride semiconductor layerand a multilayer body) 10, a multilayer body 120, a second n-type bufferlayer (n-type buffer layer between a multilayer body and alight-emitting layer) 13, a light-emitting layer 14, and p-type nitridesemiconductor layers 16, 17, and 18. The second n-type buffer layer 13is optional.

The first n-type buffer layer 10, the multilayer body 120, the secondn-type buffer layer 13, the light-emitting layer, the p-type nitridesemiconductor layers 16, 17, 18 and part of the n-type contact layer 8are etched to form a mesa portion 30. There is a p-side electrode 25 onthe top surface of the p-type nitride semiconductor layer 18 with atransparent electrode 23 therebetween. Outside the mesa portion 30 (theright side of FIG. 1 (a)), there is an n-type electrode 21 on an exposedsurface of the n-type contact layer 8. The transparent protection film27 covers the transparent electrode 23 and the etch-exposed sides oflayers. The n-side electrode 21 and the p-side electrode 25 are exposed,not covered with the transparent protection film 27.

The substrate 3, the buffer layer 5, the underlying layer 7, and then-type contact layer 8 are preferably formed by known techniques. Theirconfigurations are not critical in the present invention and thereforenot described in detail hereinafter. Their configurational parameters,such as material, composition, formation process and conditions,thickness, and impurity concentration, may be variously combined withthose in known technologies.

The two-dimensional structure of the nitride semiconductorlight-emitting element 1, illustrated in FIG. 2, can be selected fromthe various known two-dimensional structures. The two-dimensionalstructure may be one that enables the flip chip connection, a method ofconnection in which the nitride semiconductor light-emitting element isconnected to a substrate upside down, unlike that in FIG. 2. Thetwo-dimensional structure of the nitride semiconductor light-emittingelement 1 is therefore not critical in the present invention and notdescribed in detail hereinafter either.

<First N-Type Buffer Layer>

The first n-type buffer layer 10 is provided between the n-type contactlayer 8 and the multilayer body 120. The n-type contact layer 8 is grownrapidly at a high temperature, whereas the light-emitting layer 14 isgrown more slowly and at a lower temperature than the n-type contactlayer 8. The manufacture of the nitride semiconductor light-emittingelement 1 therefore involves a switch from high to low for thetemperature at which the nitride semiconductor layers are grown and fromfast to slow for the growth rate. During this switch, the first n-typebuffer layer 10 serves as a buffer layer.

The first n-type buffer layer 10 is grown at a lower temperature andmore slowly than the n-type contact layer 8, and this gives it a growthsurface (top surface) smoother than that of the n-type contact layer 8.However, the first n-type buffer layer 10 is considered to havesubstantially no effect in reducing dislocations or othercrystallographic defects.

The first n-type buffer layer 10 is grown at a lower temperature thanthe n-type contact layer 8. In the first n-type buffer layer 10, thus,some dislocations start to form what is called V-pits, according toobservations. Preferably, the first n-type buffer layer 10 lies incontact with the multilayer body 120. This improves the controllabilityof the V-pit structure (the V-pit structure reduces the influence ofthreading dislocations).

The first n-type buffer layer 10 preferably has a thickness of 50 nm orless. This limits the waviness of the growth surface (top surface) ofthe first n-type buffer layer 10. It is more preferred that thethickness of the first n-type buffer layer 10 be 5 nm or more, even morepreferably 10 nm or more. This gives the first n-type buffer layer 10 asmooth growth surface.

The first n-type buffer layer 10 preferably has an n-type impurityconcentration of 3×10¹⁸ cm⁻³ or more and 1.1×10¹⁹ cm⁻³ or less. Too highan n-type impurity concentration of the first n-type buffer layer 10 cancause low luminous efficiency at the light-emitting layer 14, which isformed above the first n-type buffer layer 10. In light of this, it ispreferred that the first n-type buffer layer 10 have an n-type impurityconcentration equal to that of the first or second semiconductor layer121 or 122 as a component of the multilayer body 120. The influence ofthe n-type impurity in the first n-type buffer layer 10 is considerednot as significant as that of the n-type impurity in the multilayer body120 because the first n-type buffer layer 10 is thinner than themultilayer body 120.

“The first n-type buffer layer 10 has an n-type impurity concentrationequal to that of the first semiconductor layer 121 as a component of themultilayer body 120” includes cases in which the n-type impurityconcentration of the first n-type buffer layer 10 is 0.85 times or moreand 1.15 times or less that of the first semiconductor layer 121. “Thefirst n-type buffer layer 10 has an n-type impurity concentration equalto that of the second semiconductor layer 122 as a component of themultilayer body 120” includes cases in which the n-type impurityconcentration of the first n-type buffer layer 10 is 0.85 times or moreand 1.15 times or less that of the second semiconductor layer 122.

It is preferred that in the first n-type buffer layer 10, the n-typeimpurity concentration be meaningfully lower than in the n-type contactlayer 8. This limits the emergence of new dislocations while helping insmoothing the growth surface of the first n-type buffer layer 10.

The first n-type buffer layer 10 is preferably an n-dopedAl_(s4)In_(t4)Ga_(1-s4-t4)N (0≤s4≤1 (more preferably 0≤s4<1) and 0≤t4≤1(more preferably 0≤t4<1)) layer. More preferably, the first n-typebuffer layer 10 is an n-doped In_(u4)Ga_(1-u4)N (0≤u4≤1, preferably0≤u4≤0.5, more preferably 0≤u4≤0.15) layer.

It is preferred that the lattice mismatch of the first n-type bufferlayer 10, which is provided between the n-type contact layer 8 and themultilayer body 120, with the n-type contact layer 8 and the secondsemiconductor layers 122 as a component of the multilayer body 120 beminimized. A greater degree of this lattice mismatch leads to a higherrisk of new crystallographic defects. It is therefore preferred that thefirst n-type buffer layer 10 have a band-gap energy equal to that of then-type contact layer or the second semiconductor layers as a componentof the multilayer body 120. For example, it is preferred that the firstn-type buffer layer 10 be an n-type GaN layer (25-nm thick).

“The first n-type buffer layer 10 has a band-gap energy equal to that ofthe n-type contact layer 8” includes cases in which the band-gap energyof the first n-type buffer layer 10 is 0.9 times or more and 1.1 timesor less that of the n-type contact layer 8. “The first n-type bufferlayer 10 has a band-gap energy equal to that of the second semiconductorlayers 122 as a component of the multilayer body 120” includes cases inwhich the band-gap energy of the first n-type buffer layer 10 is 0.9times or more and 1.1 times or less that of the second semiconductorlayers 122.

<Multilayer Body>

Through extensive research, the inventors found that a multilayer body120 provided between the first n-type buffer layer 10 and thelight-emitting layer 14 ensures that the crystal quality of the layersformed on the multilayer body 120 (e.g., the light-emitting layer 14)remains high. This is considered to ensure that luminous efficiencyremains high during high-temperature or high-rate driving. The followingdescribes the configuration of the multilayer body 120.

The multilayer body 120 has at least one stack of first and secondsemiconductor layers 121 and 122. “Stack of first and secondsemiconductor layers 121 and 122” includes cases in which a stack hastwo or more first semiconductor layers 121 and two or more secondsemiconductor layers 122, besides the case in which a stack is composedof one first semiconductor layer 121 and one second semiconductor layer122. When a stack has two or more first semiconductor layers 121 and twoor more second semiconductor layers 122, the first semiconductor layers121 alternate with the second semiconductor layers 122 to form thestack. Specifically, in the multilayer body 120, first semiconductorlayers 121 are stacked alternately with second semiconductor layers 122having a greater band-gap energy than the first semiconductor layers121, with each first semiconductor layer 121 on the first n-type bufferlayer 10 side.

The first and second semiconductor layers 121 and 122 each have athickness t₁ or t₂ of more than 10 nm and 30 nm or less. In applicationsin which luminous efficiency at room temperature is a high priority, thefirst semiconductor layer 121 has a thickness t₁ of more than 10 nm and30 nm or less, and the second semiconductor layer 122 has a thickness t₂of more than 10 nm and 40 nm or less. This ensures that threadingdislocations occurring under the multilayer body 120 are deflected atthe interface between the first and second semiconductor layers 121 and122. The lowered density of threading dislocations in the light-emittinglayer 14 ensures that the crystal quality of the light-emitting layer 14remains high. As a result, the light-emitting characteristics of thenitride semiconductor light-emitting element 1 is further improved. Forexample, luminous efficiency remains high during high-temperature orhigh-rate driving. Preferably, the first and second semiconductor layers121 and 122 each have a thickness t₁ or t₂ of 15 nm or more and 30 nm orless. In applications in which luminous efficiency at room temperatureis a high priority, it is preferred that the first semiconductor layer121 have a thickness t₁ of 15 nm or more and 30 nm or less with thesecond semiconductor layer 122 having a thickness t₂ of 15 nm or moreand 40 nm or less. First and second semiconductor layers 121 and 122having a thickness t₁ or t₂ exceeding 30 nm may affect the planarity ofthe growth surface (top surface) of the multilayer body 120. Anobservation of a cross-sectional TEM (Transmission Electron Microscope)image of the multilayer body 120 gives the thicknesses t₁ and t₂ of thefirst and second semiconductor layers 121 and 122.

It is preferred that the first and second semiconductor layers 121 and122 each contain an n-type impurity. This further reduces the density ofthreading dislocations in the light-emitting layer 14. The reason theinventors consider is as follows. Adding an n-type impurity to the firstsemiconductor layer 121 changes the lattice constants of the Group IIInitride semiconductor crystal that forms the first semiconductor layer121. Likewise, adding an n-type impurity to the second semiconductorlayer 122 changes the lattice constants of the Group III nitridesemiconductor crystal that forms the second semiconductor layer 122.These changes make the interface between the first and secondsemiconductor layers 121 and 122 more effective in deflecting threadingdislocations that occur under the multilayer body 120. As a result, thedensity of threading dislocations in the light-emitting layer 14 isfurther reduced. The n-type impurity concentration of each of the firstand second semiconductor layers 121 and 122 is preferably 3×10¹⁸ cm⁻³ ormore and less than 1.1×10¹⁹ cm⁻³, more preferably 6×10¹⁸ cm⁻³ and lessthan 1×10¹⁹ cm⁻³.

The first and second semiconductor layers 121 and 122 may have differentn-type impurity concentration s, but preferably have equal n-typeimpurity concentration s. This helps in controlling the compositions orthicknesses of the first and second semiconductor layers 121 and 122.“The first and second semiconductor layers 121 and 122 have equal n-typeimpurity concentration s” means that the n-type impurity concentrationof the first semiconductor layer 121 is 0.85 times or more and 1.15times or less that of the second semiconductor layer 122.

The inventors studied the relative excitation intensity ofphotoluminescence (PL) with different n-doping (Si) levels of the firstand second semiconductor layers 121 and 122 (both 12-nm thick), alongwith the luminous intensity of electroluminescence (EL) and itstemperature dependence. The results are illustrated in FIGS. 3 (a) and 3(b). The graph in FIG. 3 (a) illustrates the Si-level dependence of therelative excitation intensity of PL emitted by the nitride semiconductorlight-emitting element 1 at the center of the nitride semiconductorlight-emitting element 1 in top view. The “relative excitation intensityof PL” is defined as follows: PL relative excitation intensity R(%)=(Ia/Ib×10)×100, where Ia is the intensity of photoluminescence at afirst intensity of excitation light, and Ib is the intensity ofphotoluminescence at a second intensity of excitation light (the secondintensity is 10 times the first intensity). In general, the PL relativeexcitation intensity R approaches 100% with higher crystal quality ofthe light-emitting layer 14.

FIG. 3 (b) illustrates data from a study in which the nitridesemiconductor light-emitting element 1 was driven with electric currentto emit light. In FIG. 3 (b), L31 represents the Si-level dependence ofthe thermal characteristics of luminous intensity (the proportion of theluminous intensity at 25° C. (luminous intensity at a wavelength of 450nm) to that at 80° C. (luminous intensity at a wavelength of 450 nm)),and L32 represents the Si-level dependence of the output power of thenitride semiconductor light-emitting element. In FIGS. 3 (a) and 3 (b),“ref.” means that the multilayer body 120 was omitted.

As illustrated in FIGS. 3 (a) and 3 (b), n-type impurity concentration sof the first and second semiconductor layers 121 and 122 of 3.1×10¹⁸cm⁻³ or more led to high relative excitation intensities of PL, improvedthermal characteristics of luminous intensity, and great output power.N-type impurity concentration s of the first and second semiconductorlayers 121 and 122 of 5.6×10¹⁸ cm⁻³ or more resulted in higher relativeexcitation intensities of PL, further improved thermal characteristicsof luminous intensity, and greater output power.

It is preferred that the thicknesses t₁ and t₂ of the first and secondsemiconductor layers 121 and 122 be equal. “The thicknesses of the firstand second semiconductor layers 121 and 122 are equal” means that thethickness t₁ of the first semiconductor layer 121 is 0.9 times or moreand 1.1 times or less the thickness t₂ of the second semiconductor layer122. Even if the multilayer body 120 is thick, this prevents any adverseeffects the large thickness of the multilayer body 120 would have on thelayers grown on the multilayer body 120 (e.g., the light-emitting layer14). For example, the decline in the crystal quality of thelight-emitting layer 14 that would be caused by the large thickness ofthe multilayer body 120 is prevented.

The specific compositions of the nitride semiconductor layers that formthe first and second semiconductor layers 121 and 122 are not critical.The first semiconductor layer 121 is preferably anAl_(x1)In_(y1)Ga_(1-x1-y1)N (0≤x1<1 and 0<y1≤1) layer, more preferably aGa_(z1)In_(1-z1)N (0<z1<1) layer.

The second semiconductor layer 122 is preferably anAl_(x2)In_(y2)Ga_(1-x2-y2)N (0≤x2<1 and 0≤y2<1) layer, more preferably aGaN layer. The multilayer body 120 is preferably a stack ofAl_(x1)In_(y1)Ga_(1-x1-y1)N (0≤x1<1 and 0<y1≤1) layers alternating withAl_(x2)In_(y2)Ga_(1-x2-y2)N (0≤x2<1 and 0≤y2<1) layers, more preferablya stack of Ga_(z1)In_(1-z1)N (0<z1<1) layers alternating with GaNlayers.

The band-gap energy of the first semiconductor layer 121 cantheoretically be any value that is 0.77 eV or more and less than 6.28eV. In practice, however, it is preferred that the band-gap energy ofthe first semiconductor layer 121 be 2.952 eV or more and 3.425 eV orless, more preferably 3.100 eV or more and 3.379 eV or less.

The band-gap energy of the second semiconductor layer 122 cantheoretically be any value that is more than 0.77 eV and 6.28 eV orless. In practice, however, it is preferred that the band-gap energy ofthe second semiconductor layer 122 be 3.024 eV or more and 3.616 eV orless, more preferably 3.289 eV or more and 3.496 eV or less.

Including In in the first semiconductor layer 121 offers the followingtwo advantages. The first advantage is that dislocations are preventedfrom reaching the light-emitting layer 14. The high abundance of In, anelement with a large atomic radius, in the first semiconductor layer 121puts a great deal of stress on the first semiconductor layer 121. As aresult, some dislocations are deflected in the first semiconductor layer121 and therefore do not reach the light-emitting layer 14.

The second advantage is enhanced planarity of the growth surface of themultilayer body 120. The reason the inventors believe is that Infunctions as a surfactant for the growth surface of the multilayer body120 during the growth of the first semiconductor layer 121 (surfactantis a collective term for things that modify the physical or chemicalproperties of the growth surface of the multilayer body 120). Enhancedplanarity of the growth surface of the multilayer body 120 combined withfewer dislocations extending toward the light-emitting layer 14 wouldlead to even higher crystal quality of the light-emitting layer 14 and,therefore, further improved light-emitting characteristics of thenitride semiconductor light-emitting element 1.

When the first semiconductor layer 121 contains In, the In compositionof the first semiconductor layer 121 is preferably lower than that ofthe light-emitting layer 14, more preferably 0.05 or less, even morepreferably about 0.04.

The multilayer body 120 preferably has two or more stacks of first andsecond semiconductor layers 121 and 122. This ensures that luminousefficiency remains even higher during high-temperature or high-ratedriving. More preferably, the multilayer body 120 has three to sevenstacks of first and second semiconductor layers 121 and 122. Thisenhances the luminous efficiency of the nitride semiconductorlight-emitting element 1 and productivity in the manufacture thereof.

An example of a multilayer body 120 is composed of five stacks of ann-type InGaN layer (first semiconductor layer) having a thickness t₁ of12 nm and an n-type GaN layer (second semiconductor layer) having athickness t₂ of 12 nm on the top surface of the first n-type bufferlayer 10. In this example, the thicknesses t₁ and t₂ of the first andsecond semiconductor layers 121 and 122 are equal across all fivestacks. However, the thicknesses t₁ and t₂ of the first and secondsemiconductor layers 121 and 122 may vary from stack to stack within therange of 10 nm to 30 nm or, in applications in which luminous efficiencyat room temperature is a high priority, on the condition that thethicknesses t₁ and t₂ of the first and second semiconductor layers 121and 122 fall within the ranges of 10 nm to 30 nm and 10 nm to 40 nm,respectively.

The above description mainly discusses a multilayer body 120 formed byone or more stacks of first and second semiconductor layers 121 and 122.The multilayer body 120 may, however, be formed by three-layer stack(s)having a layer different from the first and second semiconductor layers121 and 122. For example, the multilayer body 120 can be formed by oneor more three-layer stacks having a 12-nm thick n-type InGaN layer(first semiconductor layer), a 12-nm thick n-type GaN layer (secondsemiconductor layer), and a third semiconductor layer in which the Inlevel is between those of the n-type InGaN and n-type GaN layers.Alternatively, the multilayer body 120 may be one in which onlyparticular structural units are three-layer stacks. The thirdsemiconductor layer may be thinner than the first and secondsemiconductor layers. For the composition and impurity concentration ofthe third semiconductor layer, the same applies as to the compositionsand impurity concentration s of the first and second semiconductorlayers.

On the multilayer body 120 there is a light-emitting layer 14. Thelight-emitting layer 14 preferably lies in contact with the multilayerbody 120, and in that case, the second semiconductor layer 122 in themultilayer body 120 closest to the light-emitting layer 14 is in contactwith the light-emitting layer 14. This simplifies the process of growingthe nitride semiconductor layers, thereby making this process morecontrollable. As a result, the yield of production of the nitridesemiconductor light-emitting element 1 is improved.

<Second N-Type Buffer Layer>

A second n-type buffer layer 13 provided between the multilayer body 120and the light-emitting layer 14 would offer the following advantages.However, the influence of providing the second n-type buffer layer 13 isconsidered not very great since the thickness of the second n-typebuffer layer 13 is similar to that of the first or second semiconductorlayer 121 or 122 (preferably, 30 nm or less).

(Case 1) Band-Gap Energy of the Second n-Type Buffer Layer 13 is Equalto or Greater than that of the Second Semiconductor Layer 122

In this case, the great band-gap energy immediately under thelight-emitting layer 14 prevents holes from leaking out. This case istherefore advantageous to light-emitting elements in which hole leakageis likely (e.g., light-emitting elements with short emissionwavelengths, such as near-ultraviolet or ultraviolet light-emittingelements). However, the drive voltage Vf tends to be high because of thehigh barrier.

(Case 2) Band-Gap Energy of the Second n-Type Buffer Layer 13 is Equalto or Smaller than that of the First Semiconductor Layer 121

In this case, the small band-gap energy immediately under thelight-emitting layer 14 leads to more efficient injection of electronsinto the light-emitting layer 14 and therefore a reduced drive voltageVf. Increasing the In content of the second n-type buffer layer 13,which leads to a lower band-gap energy of the second n-type buffer layer13, makes the crystallographic surface of the second n-type buffer layer13 even smoother, thereby enhancing the luminous efficiency of thelight-emitting layer 14 to some extent. In the second n-type bufferlayer 13, however, the increased stress can cause new defects to begenerated. The margin of production of the second n-type buffer layer 13thus tends to be small.

(Case 3) Band-Gap Energy of the Second n-Type Buffer Layer 13 is Smallerthan that of the Second Semiconductor Layer 122 and Greater than that ofthe First Semiconductor Layer 121

The structure in case 3, which is an intermediate of those in cases 1and 2, helps in ensuring a balance between the characteristics of thenitride semiconductor light-emitting element 1 and the margin ofproduction of the second n-type buffer layer 13.

In the optimization of the structure of the multilayer body 120, theinventors used AFM images of the wafer surface taken immediately beforeforming the light-emitting layer 14 (see FIG. 4) as a guide. If therewas a second n-type buffer layer 13, an AFM image of the growth surfaceof the second n-type buffer layer 13 served as a guide. If there was nosecond n-type buffer layer 13, an AFM image of the growth surface of themultilayer body 120 served as a guide. The pattern of black hexagonsseen in FIG. 4 is what is called V-pits. The inventors assume thatdislocations extending from the layers on the substrate 3 side withrespect to the first n-type buffer layer 10 toward the first n-typebuffer layer 10 emerged as visible hexagonal pyramidal cavities throughthe first n-type buffer layer 10, the multilayer body 120, and thesecond n-type buffer layer 13. The lower the density of the V-pits thebetter. Preferably, the density of the V-pits is 3×10⁸ cm⁻² or less,more preferably 0.8×10⁸ cm⁻² or less.

In improving the light-emitting characteristics or yield of the nitridesemiconductor light-emitting element 1, the size of the V-pits isimportant. The optimum size of the V-pits varies according to theconditions under which the light-emitting layer 14 is formed or underwhich the p-type nitride semiconductor layers 16, 17, and 18 are formed,and is always difficult to determine. In general, the size of the V-pitsincreases proportionally to the thickness of the first n-type bufferlayer 10, the multilayer body 120, or the second n-type buffer layer 13.If the light-emitting layer 14 and the p-type nitride semiconductorlayers 16, 17, and 18 are formed under fixed conditions, the size of theV-pits can be controlled by changing the number of layers in themultilayer body 120 (e.g., the numbers of first and second semiconductorlayers 121 and 122). The size of the V-pits also mainly depends on theconditions under which the multilayer body 120 is grown. In the presentinvention, the size of the V-pits is controlled to 70 nm to 100 nm asmeasured immediately before the formation of the light-emitting layer14.

Such a second n-type buffer layer 13 is preferably anAl_(x3)In_(y3)Ga_(1-x3-y3)N (0≤x3<1 and 0≤y3<1) layer, more preferablyan Al_(x3)In_(y3)Ga_(1-x3-y3)N (0≤x3≤0.1 and 0≤y3≤0.2) layer.

The n-type impurity concentration of the second n-type buffer layer 13is preferably 3×10¹⁸ cm⁻³ or more and less than 1.1×10¹⁹ cm⁻³. Too highan n-type impurity concentration of the second n-type buffer layer 13can cause low luminous efficiency at the light-emitting layer 14, whichis formed on the second n-type buffer layer 13. In light of this, it ispreferred that the first n-type buffer layer 10 have an n-type impurityconcentration equal to that of the first or second semiconductor layer121 or 122 as a component of the multilayer body 120.

“The second n-type buffer layer 10 has an n-type impurity concentrationequal to that of the first semiconductor layer 121 as a component of themultilayer body 120” includes cases in which the n-type impurityconcentration of the second n-type buffer layer 13 is 0.85 times or moreand 1.15 times or less that of the first semiconductor layer 121. “Thesecond n-type buffer layer 13 has an n-type impurity concentration equalto that of the second semiconductor layer 122 as a component of themultilayer body 120” includes cases in which the n-type impurityconcentration of the second n-type buffer layer 13 is 0.85 times or moreand 1.15 times or less that of the second semiconductor layer 122.

The AFM image in FIG. 4 is from a combination of an In_(0.04)Ga_(0.96)Nlayer with t₁=12 nm and a GaN layer with t₂=12 nm. This structure wasfound to be suitable for LEDs focused on good thermal characteristicsbut not ideal for LEDs focused on room-temperature characteristics. TheAFM image in FIG. 10 is from a structure composed of four pairs ofIn_(0.04)Ga_(0.96)N layers with t₁=12 nm and GaN layers with t₂=30 nmand is an AFM observation of the wafer surface taken immediately beforeforming the light-emitting layer 14. This light-emitting element alsohad a 60-nm GaN layer and a 12-nm InGaN layer as the first and secondn-type buffer layers 10 and 13, respectively, and the first n-typebuffer layer 10, the multilayer body 120, and the second n-type bufferlayer 13 were all doped with Si at 7×10¹⁸/cm³. The V-pits in FIG. 10 areapproximately 200 nm across, large compared with those in FIG. 4. Thedensity of approximately 1.5×10⁸/cm² is slightly lower than that in FIG.4 but the inventors believe falls within the range of variations at thepoint of measurement. Regarding the planarity of the plane, almostequally spaced smooth lines run winding up and down in the drawing likethose in FIG. 4, indicating that a clear step-growth surface was formed.

As discussed hereinafter, for LEDs focused on characteristics at roomtemperature, it is preferred that the crystal surface on which theactive layer is to be formed have such relatively large V-pits. V-pitswith sizes of approximately 100 nm to 300 nm, preferably 150 nm to 250nm, are suitable. Although relating to the structure of the active layerto be formed, too, there is an optimum size of V-pits for improvedoptical power. This has not been fully explained. The inventors presumethat when the V-pits are small, hole injection from V-pit sidewalls intothe quantum well layers in the flat portion is sufficiently unlikely,and when the V-pits are large, the associated irregularity of thecrystal growth surface degrades the crystallinity of well layers andaffects light-emitting characteristics.

For LEDs focused on characteristics at room temperature, it is preferredthat the V-pits reach the multilayer body at the bottom of the V-shapethereof (the bottom of the V-pits before the growth of thelight-emitting layer, i.e., the V-pits on the surface of the grownsecond n-type buffer layer 13; corresponding to the lower apexes of theV-shaped areas of the light-emitting layer in cross-sectional view ofthe completed epilayers). For LEDs focused on characteristics at roomtemperature, it is preferred that the V-pits be present as a largenumber of scattered cavities in plan view of the top portion of thelight-emitting layer with the plane surface density of the V-pits (V-pitdensity) being 1×108/cm2 or more. When the LED is focused on roomtemperature characteristics, the V-pit density need not be low and maybe higher. Even V-pit densities roughly five times higher than that inFIG. 10 provide sufficiently efficient LEDs.

<Light-Emitting Layer>

If there is a second n-type buffer layer 13, the light-emitting layer 14lies in contact with the second n-type buffer layer 13. Specifically,the initial well layer 14WI is in contact with the second n-type bufferlayer 13. If there is no second n-type buffer layer 13, thelight-emitting layer 14 lies in contact with the multilayer body 120.Specifically, the initial well layer 14WI is in contact with the(uppermost) second semiconductor layer 122 of the multilayer body 120.

The light-emitting layer 14 may have the single quantum well structure,but preferably has the multiple quantum well structure, a structure inwhich well layers 14W are stacked alternately with barrier layers 14A(e.g., see FIG. 5). The light-emitting layer 14 may have a layeredstructure in which a well layer 14W, a barrier layer 14A, and one ormore semiconductor layers different from the well and barrier layers 14Wand 14A are stacked in order.

The light-emitting layer 14 is preferably an undoped layer. Thiseffectively prevents the occurrence of new defects in the light-emittinglayer 14. Although not for sure, the inventors assume this can beexplained by a decrease in the strain the multilayer body 120 puts onthe light-emitting layer 14. “The light-emitting layer 14 is an undopedlayer” means that none of the initial well layer 14WI, the well layers14W, the final well layer 14WF, and all barrier layers 14A in thelight-emitting layer 14 is intentionally doped with an n-type or p-typeimpurity. In these layers, the n-type impurity concentration is 1×10¹⁷cm⁻³ or less, and the p-type impurity concentration is 1×10¹⁷ cm⁻³ orless.

During the growth of the p-type nitride semiconductor layers 16, 17, and18, the p-type impurity may be doped out of the p-type nitridesemiconductor layers 16, 17, and 18 into the well or barrier layers 14Aon the p-type nitride semiconductor layer 16 side through thermaldiffusion.

In the light-emitting layer 14, it is preferred that the thickness of astack of one well layer 14W and one barrier layer 14A (total thicknessof the well layer 14W and the barrier layer 14A) be 5 nm or more and 100nm or less.

(Well Layers)

The well layers 14W are formed of a Group III nitride semiconductor thatpreferably has a composition adjusted according to the emissionwavelength required of the nitride semiconductor light-emitting element1, more preferably Al_(c)Ga_(d)In_(1-c-d)N (0≤c<1 and 0<d≤1). Al-freecompositions In_(e)Ga_(1-e)N (0<e≤1) can also be used. For the emissionof, for example, ultraviolet light, or light of wavelengths of 375 nm orless, it is preferred that the well layers 14W contain Al because theband-gap energy of the well layers 14W needs to be large.

The well layers on the p-type nitride semiconductor layer 16 side arepreferably as impurity-free as possible. In other words, it is preferredto avoid introducing impurity raw materials during the growth of thewell layers on the p-type nitride semiconductor layer 16 side. Thisincreases the luminous efficiency of the nitride semiconductorlight-emitting element 1 by making nonradiative recombination lesslikely in the well layers on the p-type nitride semiconductor layer 16side. The well layers on the multilayer body 120 side preferably containan n-type impurity. This reduces the drive voltage of the nitridesemiconductor light-emitting element 1.

The well layers 14W are preferably formed of a common composition ofGroup III nitride semiconductor and preferably have equal thicknesses.This ensures that the well layers 14W have equal quantum levels andtherefore emit light of the same wavelength through electron-holerecombination. As a result, the nitride semiconductor light-emittingelement 1 has a narrow emission spectrum.

Intentionally forming the well layers 14W from varying compositions ofGroup III nitride semiconductors or giving varying thicknesses to thewell layers 14W makes the emission spectrum of the nitride semiconductorlight-emitting element 1 broad. When the nitride semiconductorlight-emitting element 1 is used for purposes such as lighting, it ispreferred that the emission spectrum of the nitride semiconductorlight-emitting element 1 be broad, and, therefore, it is preferred toform the well layers 14W from varying compositions of Group III nitridesemiconductors or give varying thicknesses to the well layers 14Wintentionally. For example, it is preferred to select appropriatethicknesses of 1 nm or more and 7 nm or less for the well layers 14W.This also offers another advantage: the luminous efficiency of thenitride semiconductor light-emitting element 1 remains high. Thethickness of the initial well layer 14WI is preferably 1 nm or more and10 nm or less.

The number of well layers 14W in the multilayer body 120 is preferably 2or more and 20 or less, more preferably 3 or more and 15 or less, evenmore preferably 4 or more and 12 or less.

(Barrier Layers)

The barrier layers 14A have a greater band-gap energy than the welllayers 14W. Specifically, the barrier layers 14A may beAl_(f)Ga_(g)In_(1-f-g)N (0≤f<1 and 0<g≤1) layers or Al-free,In_(h)Ga_(1-h)N (0<h≤1 and e>h) layers. However, it is preferred thatthe barrier layers 14A be Al_(f)Ga_(g)In_(1-f-g)N (0≤f<1 and 0<g≤1)layers because Al_(f)Ga_(g)In_(1-f-g)N (0≤f<1 and 0<g≤1) has latticeconstants substantially the same as those of the material that forms thewell layers 14W.

More preferably, the barrier layers 14A are Al_(f)In_(g)Ga_(1-f-g)N(0≤f≤0.01 and 0≤g≤0.01) layers. This effectively prevents the occurrenceof new defects in the light-emitting layer 14 by reducing the strain themultilayer body 120 puts on the light-emitting layer 14. As a result,the light-emitting characteristics of the light-emitting layer 14 isimproved.

Each barrier layer 14A can have any thickness, but preferably 1 nm ormore and 10 nm or less, more preferably 3 nm or more and 7 nm or less.The drive voltage of the nitride semiconductor light-emitting element 1decreases with smaller thickness of each barrier layer 14A. However,barrier layers 14A each having a thickness of less than 1 nm tend toresult in low luminous efficiency of the nitride semiconductorlight-emitting element 1.

Each barrier layer 14A can have any n-type impurity concentration, andit is preferred to select an appropriate level as necessary. Eachbarrier layer 14A may be an undoped layer, or may alternatively containan n-type impurity. The barrier layers 14A on the multilayer body 120side preferably contain an n-type impurity. The barrier layers 14A onthe p-type nitride semiconductor layer 16 side preferably contain alower level of n-type impurity than those on the multilayer body 120side or are not intentionally doped with an n-type impurity.

In LEDs focused on characteristics at room temperature, the barrierlayers 14A tend to be thick compared with those in LEDs focused onthermal characteristics, preferably having a thickness of 4 nm or moreand 15 nm or less, more preferably 6 nm or more and 13 nm or less. Ithas generally been speculated that when characteristics at roomtemperature is a high priority, hole injection occurs through thesidewalls of the V-pits into the well layers. It therefore appears thatthis is because even thick barrier layers are unlikely to interfere withhole injection since their thickness relative to the sidewalls of theV-pits is thin compared with that in the flat portion. Thicker barrierlayers are considered more advantageous in that they improve thecrystallinity of the well layers.

<P-Side Intermediate Layer>

The final well layer 14WF preferably has the p-type nitridesemiconductor layer 16 with a p-side intermediate layer (e.g., the layer145 in FIG. 5) therebetween. This prevents, during the growth of thep-type nitride semiconductor layers 16, 17, and 18, the p-type impurityfrom being doped out of the p-type nitride semiconductor layers 16, 17,and 18 into the light-emitting layer 14 through thermal diffusion. It isthus preferred that the thickness of the p-side intermediate layer besuch that p-type impurity does not diffuse to the final well layer 14WF,preferably less than 10 nm, more preferably less than 5 nm. Thethickness of the p-side intermediate layer may be similar to that of thebarrier layers 14A.

At least part of the p-side intermediate layer may be doped with ann-type impurity. This improves the luminous efficiency of the nitridesemiconductor light-emitting element 1.

The p-side intermediate layer is preferably an Al_(s5)Ga_(1-s5)N(0≤s5<1) layer, more preferably a GaN layer or an AlGaN layer that hasan Al composition similar to or greater than that of the barrier layers14A. In fact, the p-type impurity diffuses from the p-type nitridesemiconductor layer 16 to the p-side intermediate layer but does notdiffuse near the interface between the p-side intermediate layer and thefinal well layer 14WF.

In LEDs focused on characteristics at room temperature, the p-sideintermediate layer 145 tends to be thick compared to that in LEDsfocused on thermal characteristics, preferably having a thickness of 4nm or more and 15 nm or less, more preferably 6 nm or more and 13 nm orless. This is the same reason described regarding the barrier layers14A.

<P-Type Nitride Semiconductor Layers>

The p-type nitride semiconductor layers 16, 17, and 18 are provided onthe light-emitting layer 14 in order. The number of p-type nitridesemiconductor layers is not limited to three; it may be two or less, andit may also be four or more. The p-type nitride semiconductor layers 16,17, and 18 are preferably p-doped Al_(s6)Ga_(t6)In_(u6)N (0≤s6≤1,0≤t6≤1, 0≤u6≤1, and s6+t6+u6≠0) layers, p-doped Al_(s6)Ga_(1-s6)N(0<s6≤0.4, preferably 0.1≤s6≤0.3) layers. For example, the p-typenitride semiconductor layer 16 is a p-type AlGaN layer, the p-typenitride semiconductor layer 17 is a p-type GaN layer, and the p-typenitride semiconductor layer 18 is a p-type GaN layer that has a p-typeimpurity concentration higher than that of the p-type nitridesemiconductor layer 17.

The p-type impurity can be of any kind, preferably Mg for example. Thecarrier concentrations in the p-type nitride semiconductor layers 16,17, and 18 are preferably 1×10¹⁷ cm⁻³ or more. The p-type impurityconcentrations (different from the carrier concentrations) of the p-typenitride semiconductor layers 16, 17, and 18 are preferably 1×10¹⁹ cm⁻³or more because the activity of the p-type impurity is approximately0.01. On the light-emitting layer 14 side of the p-type nitridesemiconductor layer 16, the p-type impurity concentration may be less1×10¹⁹ cm⁻³.

The total thickness of the p-type nitride semiconductor layers 16, 17,and 18 is not critical and preferably is 30 nm or more and 300 nm orless. Thin p-type semiconductor layers 16, 17, and 18 prevent the p-typeimpurity from diffusing into the light-emitting layer 14 during theirgrowth because they need only short periods of heating to grow.

<N-Side Electrode, Transparent Electrode, and P-Side Electrode>

The n-side electrode 21 and the p-side electrode 25 are used to supplydrive power to the nitride semiconductor light-emitting element 1. FIG.2 illustrates a configuration of the n-side and p-side electrodes 21 and25 in which a pad electrode portion alone serves as an electrode.However, elongated protrusions (branch electrodes) for the diffusion ofcurrent may be connected to the n-side and p-side electrodes 21 and 25illustrated in FIG. 2. Under the p-side electrode 25 there is preferablyan insulating layer for preventing current from being injected into thep-side electrode 25. This prevents the light emitted by thelight-emitting layer 14 from being blocked by the p-side electrode 25.

The n-side electrode 21 preferably has a layered structure in whichtitanium, aluminum, and gold layers, for example, are stacked in thisorder. It is preferred that the thickness of the n-side electrode 21 be1 μm or more assuming that the n-side electrode 21 may be used for wirebonding in some cases.

The p-side electrode 25 preferably has a layered structure in whichnickel, aluminum, titanium, and gold layers, for example, are stacked inthis order, but may be made of the same material(s) as the n-sideelectrode 21. It is preferred that the thickness of the p-side electrode25 be 1 μm or more assuming that the p-side electrode 25 may be used forwire bonding in some cases.

The transparent electrode 31 is preferably made of a transparentconductive material, such as ITO (Indium Tin Oxide) or IZO (Indium ZincOxide), and preferably has a thickness of 20 nm or more and 200 nm orless.

<Production of the Nitride Semiconductor Light-Emitting Element>

The following describes an example of a method for the production of thenitride semiconductor light-emitting element 1. In the following, a“growth temperature” refers to the temperature of the substrate 3 atwhich the layer is crystallographically grown.

(Growth of an Underlying Layer)

After a buffer layer 5 is formed on the top surface of a substrate 3 by,for example, sputtering, an underlying layer 7 is formed on the topsurface of the buffer layer 5 by, for example, MOCVD (Metal OrganicChemical Vapor Deposition).

Specifically, an underlying layer 7 is grown on the substrate 3 with thebuffer layer 5 thereon in an MOCVD system, preferably at 800° C. or moreand 1250° C. or less, more preferably at 900° C. or more and 1150° C. orless. This ensures the resulting underlying layer 7 has fewcrystallographic defects and superior crystal quality. The underlyinglayer 7 is preferably an undoped layer and is preferably grown to athickness of approximately 2 to 5 μm.

(Growth of an N-Type Contact Layer)

An n-type contact layer 8 is then formed on the top surface of theunderlying layer 7 by, for example, MOCVD. Specifically, an n-typecontact layer 8 is grown in the MOCVD system, preferably at 800° C. ormore and 1250° C. or less, more preferably at 900° C. or more and 1150°C. or less. This ensures the resulting n-type contact layer 8 has fewcrystallographic defects and superior crystal quality. The reactant gascontains, for example, silane gas (SiH₄), and the amount of silane gasis preferably adjusted to make the Si level approximately 1×10¹⁹/cm³. Itis preferred that the n-type contact layer 8 be grown to a thickness ofapproximately 1 to 4 μm.

(Growth of a First N-Type Buffer Layer)

A first n-type buffer layer 10 is then formed on the top surface of then-type contact layer 8 by, for example, MOCVD. Specifically, after thetemperature inside of the MOCVD system is lowered, a first n-type bufferlayer 10 is grown with the growth rate controlled to a slower speed.

Alternatively, the substrate 3 on which the n-type contact layer 8 andall lower layers have been formed may be taken out of a first MOCVDsystem, exposed to atmospheric air, and then put into a second MOCVDsystem for the formation of the first n-type buffer layer 10 andsubsequent layers. In this approach, the system for growing theunderlying and n-type contact layers 7 and 8, which are thick layers(requiring fast growth), can be different from that for growing thelight-emitting layer 14 (requiring slow growth and growth with highuniformity in crystal quality). The manufacturer can therefore selectthe most suitable film formation system for the growth of each layer,and this improves efficiency in the production of the nitridesemiconductor light-emitting element 1.

Specifically, the growth temperature for the first n-type buffer layer10 is preferably 950° C. or less, more preferably 700° C. or more, evenmore preferably 750° C. or more. A growth temperature for the firstn-type buffer 10 of 700° C. or more ensures that luminous efficiencyremains high at the light-emitting layer 14.

(Growth of a Multilayer Body)

A multilayer body 120 is then formed on the top surface of the firstn-type buffer layer 10 by, for example, MOCVD. The growth temperaturefor the multilayer body 120 is preferably equal to or lower than thatfor the first n-type buffer layer 10.

For high and sustained film quality of the multilayer body 120, it ismore preferred that the growth temperature for the multilayer body 120be 600° C. or more, even more preferably 700° C. or more. The firstn-type buffer layer 10 and the multilayer body 120 may be grown at equalgrowth temperatures.

(Growth of a Second N-Type Buffer Layer)

A second n-type buffer layer 13 can be grown under the same conditionsas in the formation of the multilayer body 12, except for thecomposition of gases supplied to the MOCVD system.

(Growth of a Light-Emitting Layer and P-Type Nitride SemiconductorLayers)

A light-emitting layer 14 and p-type nitride semiconductor layers 16,17, and 18 are then formed on the top surface of the multilayer body 120in order following known methods.

The following raw-material gases can be used in the MOCVDcrystallographic growth of layers. The Ga raw-material gas can be TMG(trimethylgallium) or TEG (triethylgallium). The Al raw-material gas canbe TMA (trimethylalluminum) or TEA (triethylaluminum). The Inraw-material gas can be TMI (trimethylindium) or TEI (triethyindium).The N raw-material gas can be NH₃ or DMH_(y) (dimethylhydrazine). Theraw-material gas for Si as an n-type impurity can be SiH₄, Si₂H₆, ororganic silicon. The raw-material gas for Mg as a p-type impurity can beCp₂Mg.

(Etching and Formation of Electrodes)

The following layers are then etched to expose part of the n-typecontact layer 8: the p-type nitride semiconductor layers 16, 17, and 18,the light-emitting layer 14, the second n-type buffer layer 13, themultilayer body 120, the first n-type buffer layer 10, and the n-typecontact layer 8. An n-side electrode 21 is formed on the top surface ofthe n-type contact layer 8 exposed through this process of etching, anda transparent electrode 23 and a p-side electrode 25 are formed on thetop surface of the p-type nitride semiconductor layer 18 in order. Thena transparent protection film 27 is formed to cover the transparentelectrode 23 and the sides of the layers exposed through the aboveetching process. In this way, a nitride semiconductor light-emittingelement 1 is obtained.

The substrate 3 may be removed. The time when to remove the substrate 3is not critical. For example, when two or more MOCVD systems are used togrow the nitride semiconductor layers, the substrate 3 can be removedbetween taking the substrate 3 out of a first MOCVD system and puttingit into a second MOCVD system.

It is also possible to grow the underlying, n-type contact, and firstn-type contact layers 7, 8, and 10 in a first MOCVD system and themultilayer body 120 and subsequent layers in a second MOCVD system.However, growing the underlying and n-type contact layers 7 and 8 in afirst MOCVD system and the first n-type buffer layer 10, the multilayerbody 120, and subsequent layers in a second MOCVD system would improvethe throughput of the second MOCVD system.

<Overall Summary of Embodiments>

The nitride semiconductor light-emitting element 1 in FIG. 1 includes atleast an n-type nitride semiconductor layer 8, a light-emitting layer14, and p-type nitride semiconductor layers 16, 17, and 18. A multilayerbody 120 is provided between the n-type nitride semiconductor layer 8and the light-emitting layer 14, and the multilayer body 120 has atleast one stack of first and second semiconductor layers 121 and 122.The second semiconductor layer 122 has a greater band-gap energy thanthe first semiconductor layer 121. Each of the first and secondsemiconductor layers 121 and 122 has a thickness of more than 10 nm and30 nm or less. This further improves the light-emitting characteristicsof the nitride semiconductor light-emitting element 1.

The room temperature characteristics-oriented nitride semiconductorlight-emitting element 1′ in FIG. 11 has a great difference from that inFIG. 1, a more dense population of large-sized V-pits 15. The firstsemiconductor layer 121 has a thickness of more than 10 nm and 30 nm orless. The second semiconductor layer 122 has a thickness of more than 10nm and 40 nm or less. This further improves the light-emittingcharacteristics of the room temperature characteristics-oriented nitridesemiconductor light-emitting element 1′.

The first semiconductor layer 121 is preferably anAl_(x1)In_(y1)Ga_(1-x1-y1)N (0≤x1<1 and 0<y1≤1) layer, and the secondsemiconductor layer 122 is preferably an Al_(x2)In_(y2)Ga_(1-x2-y2)N(0≤x2<1 and 0≤y2<1) layer.

Each of the first and second semiconductor layers 121 and 122 preferablyhas an n-type impurity concentration of 3×10¹⁸ cm⁻³ or more and lessthan 1.1×10¹⁹ cm⁻³. This further reduces the density of threadingdislocations in the light-emitting layer 14.

The first and second semiconductor layers 121 and 122 preferably haveequal n-type impurity concentrations. This helps in controlling thecompositions or thicknesses of the first and second semiconductor layers121 and 122.

The first and second semiconductor layers 121 and 122 preferably haveequal thicknesses. This further enhances the crystal quality of thelight-emitting layer 14.

The multilayer body 120 preferably has three to seven stacks of thefirst and second semiconductor layers 121 and 122. This enhances theluminous efficiency of the nitride semiconductor light-emitting element1 and productivity in the manufacture thereof.

It is preferred that the second semiconductor layer 122 in themultilayer body 120 closest to the light-emitting layer 14 be in contactwith the light-emitting layer 14. This improves the yield of productionof the nitride semiconductor light-emitting element 1 and simplifies theprocess of growing the nitride semiconductor layers.

It is preferred that a second n-type buffer layer 13 be provided betweenthe multilayer body 120 and the light-emitting layer 14. Preferably, thesecond n-type buffer layer 13 is an Al_(x3)In_(y3)Ga_(1-x3-y3)N (0≤x3<1and 0≤y3<1) layer that contains an n-type impurity and lies in contactwith the light-emitting layer. This improves the yield of production ofthe nitride semiconductor light-emitting element 1 by allowing themanufacturer to optimize the structure of the nitride semiconductorlight-emitting element 1 to meet the emission wavelength or operationvoltage specification.

The band-gap energy of the second n-type buffer layer 13 is preferablyequal to or greater than that of the second semiconductor layer 122.This prevents holes from leaking out.

The band-gap energy of the second n-type buffer layer 13 is preferablysmaller than that of the second semiconductor layer 122 and greater thanthat of the first semiconductor layer 121. This ensures a balancebetween the light-emitting characteristics of the nitride semiconductorlight-emitting element 1 and the margin of production of the secondn-type buffer layer 13.

The band-gap energy of the second n-type buffer layer 13 is preferablyequal to or smaller than that of the first semiconductor layer 121. Thisleads to more efficient injection of electrons into the light-emittinglayer 14.

The thickness of the second n-type buffer layer is preferably 30 nm orless. This prevents the occurrence of faults due to the presence of thesecond n-type buffer layer 13.

It is preferred that a first n-type buffer layer 10 be provided betweenthe n-type nitride semiconductor layer 8 and the multilayer body 120.Preferably, the first n-type buffer layer 10 is anAl_(s4)In_(t4)Ga_(1-s4-t4)N (0≤s4<1 and 0≤t4<1) layer that contains ann-type impurity and lies in contact with the multilayer body 120. Thisimproves the controllability of the V-pit structure.

The band-gap energy of the first n-type buffer layer 10 is preferablyequal to that of the second semiconductor layer 122. This prevents theoccurrence of new crystallographic defects.

The n-type impurity concentration of the first n-type buffer layer 10 ispreferably equal to at least one of the n-type impurity concentrationsof the first and second semiconductor layers 121 and 122. This enhancesthe luminous efficiency at the light-emitting layer 14.

The thickness of the first n-type buffer layer 10 is preferably 50 nm orless. This prevents a decline in the luminous efficiency of the nitridesemiconductor light-emitting element 1 by limiting the waviness of thegrowth surface of the first n-type buffer layer 10.

The light-emitting layer 14 is preferably an undoped layer. Thisprevents the occurrence of new defects in the light-emitting layer 14.

The light-emitting layer 14 preferably has the single quantum wellstructure or a multiple quantum well structure in which well layers arestacked alternately with Al_(f)In_(g)Ga_(1-f-g)N (0≤f≤0.01 and 0≤g≤0.01)barrier layers. This effectively prevents the occurrence of new defectsin the light-emitting layer 14.

EXAMPLES

The following describes the present invention in more detail byproviding some examples. However, the present invention is not limitedto these examples.

Example 1

In Example 1, nitride semiconductor light-emitting elements having theenergy band diagram illustrated in FIG. 5 were produced.

(Preparation of a Substrate (Wafer))

A wafer that was a 100-mm diameter sapphire substrate was prepared. Thewafer had a textured top surface formed by projections 3 a alternatingwith recesses 3 b. Such a textured profile was formed following themethod presented below.

First, a mask with a two-dimensional pattern of the projections 3 a inFIG. 1 (a) defined thereon was placed on the wafer. The wafer topsurface was then dry-etched using this mask. Some areas were dry-etchedaway to leave the recesses 3 b, and the other areas were not dry-etchedand left as the projections 3 a. That is, the projections 3 a werearranged in lines in the following directions: <11-20> on the wafer topsurface, +60° to <11-20> on the wafer top surface, and −60° to <11-20>on the wafer top surface. The projections 3 a on the wafer top surfacewere at the apexes of triangles and periodically arranged along thethree sides of the triangles.

The projections 3 a on the wafer top surface were round in shape, withthe diameter being approximately 1.2 μm. The interval between adjacentprojections 3 a (side length of the triangles) based on apexes was 2 μm,and the height of the projections 3 a was approximately 0.6 μm. Theprojections 3 a had the side-view shape illustrated in FIG. 1 (a), withrounded tops. The recesses 3 b had the side-view shape illustrated inFIG. 1 (a).

(Formation of a Buffer Layer)

After the formation of the projections 3 a and recesses 3 b, the wafertop surface was subjected to an RCA clean. The RCA-cleaned wafer was putinto the chamber of a reactive sputtering system, and an aluminumnitride buffer layer 5 (25-nm thick) was formed. The resulting bufferlayer 5 was an aggregate of columnar crystals extending normal to thewafer top surface and uniform in grain size.

(Growth of Underlying and N-Type Contact Layers)

The wafer with the buffer layer 5 thereon was put into an MOCVD system,and an undoped GaN underlying layer 7 was crystallographically grown.The underlying layer 7 was 4.5 μm thick.

A Si-doped n-type GaN layer (n-type contact layer 8) was thencrystallographically grown on the top surface of the underlying layer 7by MOCVD. The n-type contact layer 8 was 4.5 μm thick and had an n-typeimpurity concentration of 1×10¹⁹ cm⁻³.

(Growth of a First N-Type Buffer Layer)

After the wafer temperature was lowered to 801° C., a 25-nm thickSi-doped GaN layer (first n-type buffer layer 10) wascrystallographically grown by MOCVD. The crystallographically grownSi-doped GaN layer was in contact with the n-type contact layer 8 andhad an n-type impurity concentration of 7.4×10¹⁸ cm⁻³.

(Growth of a Multilayer Body)

With the wafer temperature maintained at 801° C., a multilayer body 120was crystallographically grown. Specifically, five stacks were formedthat were each composed of a 12-nm thick Si-doped InGaN (In composition,0.04) layer and a 12-nm thick Si-doped GaN layer with the former on theside touching the first n-type buffer layer 10. In all layers of themultilayer body 120, the n-type impurity concentration was 7.4×10¹⁸cm⁻³. This structure omits the second n-type buffer layer 13.

(Growth of a Light-Emitting Layer)

The wafer temperature was lowered to 672° C. Then well layers 14W werecrystallographically grown alternately with barrier layers 14A on thetop surface of the multilayer body 120 to form a light-emitting layer14.

Well layers 14W (eight layers) were crystallographically grown usingnitrogen gas as carrier gas. The crystallographically grown well layers14W, initial well layer 14WI, and final well layer 14WF were undopedIn_(x)Ga_(1-x)N (x=0.20) layers. As a result, the wavelength of thephotoluminescence from the well layers 14W was 448 nm. The well layers14W and the initial well layer 14WI were each made to a thickness of3.38 nm, and the final well layer 14WF was made to a thickness of 5.0nm.

Barrier layers 14A (seven layers) were crystallographically grownalternately with the well layers 14W. The crystallographically grownbarrier layers 14A were undoped Al_(y)Ga_(1-y)N (y=0.001) layers andwere 4.0 nm thick.

(Growth of a P-Side Intermediate Layer)

An undoped AlGaN (Al composition, 0.001) p-side intermediate layer 145(3.0-nm thick) was crystallographically grown on the top surface of thefinal well layer 14WF.

(Growth of P-Type Nitride Semiconductor Layers)

The wafer temperature was increased to 1000° C. Then a p-typeAl_(0.18)Ga_(0.82)N layer (p-type nitride semiconductor layer 16;thickness, 9 nm; p-type impurity concentration, 2×10¹⁹ cm⁻³), a p-typeGaN layer (p-type nitride semiconductor layer 17; thickness, 20 nm;p-type impurity concentration, 3×10¹⁹ cm⁻³), and a p-type contact layer(p-type nitride semiconductor layer 18; thickness, 7 nm; p-type impurityconcentration, 1×10²⁰ cm⁻³) were crystallographically grown on the topsurface of the p-side intermediate layer 145 in order.

In the crystallographic growth of these layers, the Ga raw-material gaswas TMG (trimethylgallium), the Al raw-material gas was TMA(trimethylalluminum), the In raw-material gas was TMI (trimethylindium),and the N raw-material gas was NH₃. The raw-material gas for Si as ann-type impurity was SiH₄, and the raw-material gas for Mg as a p-typeimpurity was Cp₂Mg.

(Etching and Formation of Electrodes)

The wafer was taken out of the MOCVD system. The following layers werethen etched to expose part of the n-type contact layer 8: the p-typecontact layer, the p-type GaN layer, the p-type Al_(0.18)Ga_(0.82)Nlayer, the p-side intermediate layer 145, the light-emitting layer 14,the multilayer body 120, the first n-type buffer layer 10, and then-type contact layer 8. A Au n-side electrode 21 was formed on the topsurface of the n-type contact layer 8 exposed through this process ofetching. An ITO transparent electrode 23 and a Au p-side electrode 25were formed on the top surface of the p-type contact layer 18 in order.A SiO₂ film (transparent protection film 27) was formed, primarilycovering the transparent electrode 23 and the sides of the layersexposed through the above etching process. The wafer was then dividedinto 620×680 μm chips. In this way, nitride semiconductor light-emittingelements of this example were obtained.

A resulting nitride semiconductor light-emitting element displayed bluelight emission with a dominant wavelength of 450 nm when operated atroom temperature with a current of 120 mA. The optical power was 170 mW,and the voltage applied was 3.05 V. The percentage of the optical powerat 80° C. to that at room temperature was 98%. A nitride semiconductorlight-emitting element produced in the same way as in this exampleexcept for the omission of the multilayer body had an optical power of161 mW, and the percentage of its optical power at 80° C. to that atroom temperature was 94%. This indicates that the presence of themultilayer body improved the light-emitting characteristics of thenitride semiconductor light-emitting element.

Example 2

In Example 2, nitride semiconductor light-emitting elements having theenergy band diagram illustrated in FIG. 6 were produced. Underlying andn-type contact layers 7 and 8 were crystallographically grown in anMOCVD system following the method described in Example 1, and then themethod presented below was followed to obtain nitride semiconductorlight-emitting elements.

(Growth of a First N-Type Buffer Layer)

With the wafer inside the MOCVD system, the wafer temperature waslowered to 801° C. A 25-nm thick Si-doped GaN layer (first n-type bufferlayer 10) was then crystallographically grown by MOCVD. Thecrystallographically grown Si-doped GaN layer was in contact with then-type contact layer 8 and had an n-type impurity concentration of7.4×10¹⁸ cm⁻³.

(Growth of a Multilayer Body)

With the wafer temperature maintained at 801° C., a multilayer body 120was crystallographically grown. Specifically, four stacks were formedthat were each composed of a 15-nm thick Si-doped InGaN (In composition,0.04) layer and an 11-nm thick Si-doped GaN layer with the former on theside touching the first n-type buffer layer 10. In all layers of themultilayer body 120, the n-type impurity concentration was 7.4×10¹⁸cm⁻³.

(Growth of a Second N-Type Buffer Layer)

A 12-nm thick AlInGaN (Al composition, 0.01; In composition, 0.04) layer(second n-type buffer layer 13) was crystallographically grown on thetop surface of the multilayer body 120 by MOCVD. Thecrystallographically grown AlInGaN layer had an n-type impurityconcentration of 7.4×10¹⁸ cm⁻³.

(Growth of a Light-Emitting Layer)

The wafer temperature was lowered to 672° C. Then well layers 14W werecrystallographically grown alternately with barrier layers 14A on thetop surface of the second n-type buffer layer 13 to form alight-emitting layer 14.

Well layers 14W (eight layers) were crystallographically grown usingnitrogen gas as carrier gas. The crystallographically grown well layers14W, initial well layer 14WI, and final well layer 14WF were undopedIn_(x)Ga_(1-x)N (x=0.20) layers. As a result, the wavelength of thephotoluminescence from the well layers 14W was 448 nm. The well layers14W and the initial well layer 14WI were each made to a thickness of3.58 nm, and the final well layer 14WF was made to a thickness of 5.0nm.

Barrier layers 14A (seven layers) were crystallographically grownalternately with the well layers 14W. The crystallographically grownbarrier layers 14A were undoped GaN layers and were 4.0 nm thick.

(Growth of a P-Side Intermediate Layer)

An undoped AlGaN (Al composition, 0.001) p-side intermediate layer 145(3.0-nm thick) was crystallographically grown on the top surface of thefinal well layer 14WF.

(Growth of P-Type Nitride Semiconductor Layers, Etching, and Formationof Electrodes)

The method described in Example 1 was followed to form p-type nitridesemiconductor layers 16, 17, and 18, carry out etching, form n-side,transparent, and p-side electrodes 21, 23, and 25 and a transparentprotective film 27, and divide the wafer into 620×680 μm chips. In thisway, nitride semiconductor light-emitting elements of this example wereobtained.

A resulting nitride semiconductor light-emitting element displayed bluelight emission with a dominant wavelength of 450 nm when operated atroom temperature with a current of 120 mA. The optical power was 171 mW,and the voltage applied was 3.04 V. The percentage of the optical powerat 80° C. to that at room temperature was 98%.

Example 3

In Example 3, nitride semiconductor light-emitting elements having theenergy band diagram illustrated in FIG. 7 were produced. After thecrystallographical growth of underlying, n-type contact, and firstn-type buffer layers 7, 8, and 10 following the methods described inExamples 1 and 2, the method presented below was followed to obtainnitride semiconductor light-emitting elements.

(Growth of a Multilayer Body)

With the wafer temperature maintained at 801° C., a multilayer body 120was crystallographically grown. Specifically, five stacks were formedthat were each composed of a 12-nm thick Si-doped InGaN (In composition,0.04) layer and a 12-nm thick Si-doped GaN layer with the former on theside touching the first n-type buffer layer 10. In all layers of themultilayer body 120, the n-type impurity concentration was 7.4×10¹⁸cm⁻³.

(Growth of a Second N-Type Buffer Layer)

A 3-nm thick AlInGaN (Al composition, 0.02; In composition, 0.005) layer(second n-type buffer layer 13) was crystallographically grown on thetop surface of the multilayer body 120 by MOCVD. Thecrystallographically grown AlInGaN layer had an n-type impurityconcentration of 7.4×10¹⁸ cm⁻³.

(Growth of a Light-Emitting Layer)

The wafer temperature was lowered to 672° C. Then well layers 14W werecrystallographically grown alternately with barrier layers 14A on thetop surface of the second n-type buffer layer 13 to form alight-emitting layer 14.

Well layers 14W (eight layers) were crystallographically grown usingnitrogen gas as carrier gas. The crystallographically grown well layers14W, initial well layer 14WI, and final well layer 14WF were undopedIn_(x)Ga_(1-x)N (x=0.20) layers. As a result, the wavelength of thephotoluminescence from the well layers 14W was 448 nm. The well layers14W and the initial well layer 14WI were each made to a thickness of3.38 nm, and the final well layer 14WF was made to a thickness of 5.0nm.

Barrier layers 14A (seven layers) were crystallographically grownalternately with the well layers 14W. The crystallographically grownbarrier layers 14A were undoped Al_(y)Ga_(1-y)N (y=0.001) layers andwere 4.0 nm thick.

(Growth of a P-Side Intermediate Layer)

An undoped Al_(y)Ga_(1-y)N (y=0.001) p-side intermediate layer 145(3.0-nm thick) was crystallographically grown on the top surface of thefinal well layer 14WF.

(Growth of P-Type Nitride Semiconductor Layers, Etching, and Formationof Electrodes)

The method described in Example 1 was followed to form p-type nitridesemiconductor layers 16, 17, and 18, carry out etching, form n-side,transparent, and p-side electrodes 21, 23, and 25 and a transparentprotective film 27, and divide the wafer into 620×680 μm chips. In thisway, nitride semiconductor light-emitting elements of this example wereobtained.

A resulting nitride semiconductor light-emitting element displayed bluelight emission with a dominant wavelength of 450 nm when operated atroom temperature with a current of 120 mA. The optical power was 169 mW,and the voltage applied was 3.07 V. The percentage of the optical powerat 80° C. to that at room temperature was 98.5%.

Example 4

In Example 4, nitride semiconductor light-emitting elements having theenergy band diagram illustrated in FIG. 8 were produced. After thecrystallographical growth of underlying, n-type contact, and firstn-type buffer layers 7, 8, and 10 following the methods described inExamples 1 and 2, the method presented below was followed to obtainnitride semiconductor light-emitting elements.

(Growth of a Multilayer Body)

With the wafer temperature maintained at 801° C., a multilayer body 120was crystallographically grown. Specifically, four stacks were formedthat were each composed of a 12-nm thick Si-doped InGaN (In composition,0.04) layer and a 12-nm thick Si-doped GaN layer with the former on theside touching the first n-type buffer layer 10. In all layers of themultilayer body 120, the n-type impurity concentration was 7.4×10¹⁸cm⁻³.

(Growth of a Second N-Type Buffer Layer)

A 12-nm thick AlInGaN (Al composition, 0.0025; In composition, 0.042)layer (second n-type buffer layer 13) was crystallographically grown onthe top surface of the multilayer body 120 by MOCVD. Thecrystallographically grown AlInGaN layer had an n-type impurityconcentration of 7.4×10¹⁸ cm⁻³.

(Growth of a Light-Emitting Layer and P-Type Nitride SemiconductorLayers, Etching, and Formation of Electrodes)

A light-emitting layer 14 and a p-side intermediate layer 15 werecrystallographically grown following the method described in Example 3.The method described in Example 1 was then followed to form p-typenitride semiconductor layers 16, 17, and 18, carry out etching, formn-side, transparent, and p-side electrodes 21, 23, and 25 and atransparent protective film 27, and divide the wafer into 620×680 μmchips. In this way, nitride semiconductor light-emitting elements ofthis example were obtained.

A resulting nitride semiconductor light-emitting element displayed bluelight emission with a dominant wavelength of 450 nm when operated atroom temperature with a current of 120 mA. The optical power was 170 mW,and the voltage applied was 3.02 V. The percentage of the optical powerat 80° C. to that at room temperature was 97.5%.

Example 5

In Example 5, nitride semiconductor light-emitting elements having theenergy band diagram illustrated in FIG. 9 were produced. Underlying andn-type contact layers 7 and 8 were grown in a first MOCVD system. Thesubstrate 3 was then taken out of the first MOCVD system and put into asecond MOCVD system, and the first n-type buffer layer 10 and subsequentlayers were grown. Specifically, the method presented below was followedto obtain nitride semiconductor light-emitting elements.

(Growth of a First N-Type Buffer Layer)

After the wafer was put into the second MOCVD system, the wafertemperature was lowered to 801° C. A 25-nm thick Si-doped GaN layer(first n-type buffer layer 10) was then crystallographically grown byMOCVD. The crystallographically grown Si-doped GaN layer was in contactwith the n-type contact layer 8 and had an n-type impurity concentrationof 9×10¹⁸ cm⁻³.

(Growth of a Multilayer Body)

With the wafer temperature maintained at 801° C., a multilayer body 120was crystallographically grown. Specifically, four stacks were formedthat were each composed of a 12-nm thick Si-doped InGaN (In composition,0.04) layer and a 12-nm thick Si-doped GaN layer with the former on theside touching the first n-type buffer layer 10. In all layers of themultilayer body 120, the n-type impurity concentration was 7.4×10¹⁸cm⁻³.

(Growth of a Second N-Type Buffer Layer)

A 12-nm thick AlInGaN (Al composition, 0.0025; In composition, 0.042)layer (second n-type buffer layer 13) was crystallographically grown onthe top surface of the multilayer body 120 by MOCVD. Thecrystallographically grown AlInGaN layer had an n-type impurityconcentration of 7.4×10¹⁸ cm⁻³.

(Growth of a Light-Emitting Layer and a P-Side Intermediate Layer)

After a light-emitting element 14 was crystallographically grownfollowing the method described in Example 1, a p-side intermediate layer145 was crystallographically grown following the method described inExample 2.

(Growth of P-Type Nitride Semiconductor Layers)

The wafer temperature was increased to 1100° C. Then a p-typeAl_(0.18)Ga_(0.82)N layer (p-type nitride semiconductor layer 16;thickness, 12 nm; p-type impurity concentration, 2×10¹⁹ cm⁻³), a p-typeGaN layer (p-type nitride semiconductor layer 17; thickness, 20 nm;p-type impurity concentration, 3×10¹⁹ cm⁻³), and a p-type contact layer(p-type nitride semiconductor layer 18; thickness, 7 nm; p-type impurityconcentration, 1×10²⁰ cm⁻³) were crystallographically grown on the topsurface of the p-side intermediate layer 145 in order.

(Etching and Formation of Electrodes)

The method described in Example 1 was followed to carry out etching,form n-side, transparent, and p-side electrodes 21, 23, and 25 and atransparent protective film 27, and divide the wafer into 620×680 μmchips. In this way, nitride semiconductor light-emitting elements ofthis example were obtained.

A resulting nitride semiconductor light-emitting element displayed bluelight emission with a dominant wavelength of 450 nm when operated atroom temperature with a current of 120 mA. The optical power was 170 mW,and the voltage applied was 3.05 V. The percentage of the optical powerat 80° C. to that at room temperature was 98%. A nitride semiconductorlight-emitting element produced in the same way as in this exampleexcept for the omission of the multilayer body had an optical power of161 mW, and the percentage of its optical power at 80° C. to that atroom temperature was 94%. This indicates that the presence of themultilayer body improved the light-emitting characteristics of thenitride semiconductor light-emitting element.

Example 6

In Example 6, nitride semiconductor light-emitting elements wereproduced that had an energy band diagram very similar to that in FIG. 7but whose emission wavelength was in the near-ultraviolet region. Inthis example, underlying, n-type contact, and first n-type buffer layers7, 8, and 10, a multilayer body 120, and a second n-type buffer layer 13were crystallographically grown following the method described inExample 3, and then the method presented below was followed to producenitride semiconductor light-emitting elements.

(Growth of a Light-Emitting Layer)

The wafer temperature was lowered to 698° C. Then well layers 14W werecrystallographically grown alternately with barrier layers 14A on thetop surface of the second n-type buffer layer 13 to form alight-emitting layer 14.

Well layers 14W (eight layers) were crystallographically grown usingnitrogen gas as carrier gas. The crystallographically grown well layers14W, initial well layer 14WI, and final well layer 14WF were undopedIn_(x)Ga_(1-x)N (x=0.10) layers. As a result, the wavelength of thephotoluminescence from the well layers 14W was 403 nm. The well layers14W and the initial well layer 14WI were each made to a thickness of3.38 nm, and the final well layer 14WF was made to a thickness of 5.0nm.

Barrier layers 14A (seven layers) were crystallographically grownalternately with the well layers 14W. The crystallographically grownbarrier layers 14A were undoped Al_(y)Ga_(1-y)N (y=0.05) layers and were4.0 nm thick.

(Growth of a P-Side Intermediate Layer)

An undoped Al_(y)Ga_(1-y)N (y=0.05) p-side intermediate layer 145(3.0-nm thick) was crystallographically grown on the top surface of thefinal well layer 14WF.

(Growth of P-Type Nitride Semiconductor Layers, Etching, and Formationof Electrodes)

The method described in Example 1 was followed to form p-type nitridesemiconductor layers 16, 17, and 18, carry out etching, form n-side,transparent, and p-side electrodes 21, 23, and 25 and a transparentprotective film 27, and divide the wafer into 440×530 μm chips. In thisway, nitride semiconductor light-emitting elements of this example wereobtained.

A resulting nitride semiconductor light-emitting element displayedviolet light emission with a peak wavelength of 405 nm when operated atroom temperature with a current of 50 mA. The optical power was 70 mW,and the voltage applied was 3.15 V. A nitride semiconductorlight-emitting element produced in the same way as in this exampleexcept for the omission of the multilayer body had an optical power of63 mW. This indicates that the presence of the multilayer body improvedthe light-emitting characteristics of the nitride semiconductorlight-emitting element.

Example 7

In Example 7, the method described in Example 2 was followed to producenitride semiconductor light-emitting elements except that thecomposition of the first n-type buffer layer 10 was different.

First, underlying and n-type contact layers 7 and 8 werecrystallographically grown in an MOCVD system following the methoddescribed in Example 1. The method presented below was then followed toproduce nitride semiconductor light-emitting elements.

(Growth of a First N-Type Buffer Layer)

With the wafer inside the MOCVD system, the wafer temperature was set to796° C. A 35-nm thick Si-doped InGaN layer (first n-type buffer layer10) was then crystallographically grown by MOCVD. Thecrystallographically grown Si-doped InGaN layer was in contact with then-type contact layer 8 and had an n-type impurity concentration of9.0×10¹⁸ cm⁻³.

The method described in Example 2 was followed to carry out the growthof a multilayer body 120, second n-type buffer, light-emitting, andp-side intermediate layers 13, 14, and 145, and p-type nitridesemiconductor layers 16, 17, and 18, etching, and the formation ofelectrodes.

A resulting nitride semiconductor light-emitting element displayed bluelight emission with a dominant wavelength of 450 nm when operated atroom temperature with a current of 120 mA. The optical power was 171 mW,and the voltage applied was 3.04 V. The percentage of the optical powerat 80° C. to that at room temperature was 98%.

Example 8

In Example 8, the method described in Example 1 was followed to producenitride semiconductor light-emitting elements except that the n-typeimpurity concentration of the multilayer body was different. Somespecifications for the nitride semiconductor light-emitting element mayrequire fine tuning of operation voltage to avoid losing yield. Aneffective way to increase the operation voltage slightly according tothe specifications for the nitride semiconductor light-emitting elementis to reduce the n-type impurity concentration of one of the two layersthat form the multilayer body.

(Growth of a Multilayer Body)

With the wafer temperature maintained at 801° C., a multilayer body 120was crystallographically grown. Specifically, four stacks were formedthat were each composed of a 12-nm thick Si-doped InGaN (In composition,0.04) layer and a 12-nm thick Si-doped GaN layer with the former on theside touching the first n-type buffer layer 10. The Si-doped GaN layerhad an n-type impurity concentration of 7.4×10¹⁸ cm⁻³. The Si-dopedInGaN layer had an n-type impurity concentration of 4×10¹⁸ cm⁻³.

A resulting nitride semiconductor light-emitting element (620×680 μmsize) displayed blue light emission with a dominant wavelength of 450 nmwhen operated at room temperature with a current of 120 mA. The opticalpower was 171 mW, and the voltage applied was 3.06 V. The percentage ofthe optical power at 80° C. to that at room temperature was 98%. Byadjusting an n-type impurity concentration in the multilayer body, theoperation voltage was successfully fine-tuned without changing theoptical power.

Example 9

In Example 9, the method described in Example 2 was followed to producenitride semiconductor light-emitting elements except that the p-sideintermediate layer 145 crystallographically grown on the top surface ofthe final well layer 14WF was an undoped GaN layer (4.0-nm thick). Inthis example, too, the p-type impurity diffused from the p-type nitridesemiconductor layer 16 to the p-side intermediate layer but did notdiffuse near the interface between the p-side intermediate layer 145 andthe final well layer 14WF.

When a resulting nitride semiconductor light-emitting element wasoperated at room temperature with a current of 120 mA, its optical powerand the voltage applied were similar to those in Example 2. Thepercentage of the optical power at 80° C. to that at room temperaturewas 98%.

Example 10

FIG. 11 is a cross-section of a room temperaturecharacteristics-oriented nitride semiconductor light-emitting elementaccording to another embodiment of the present invention. FIG. 12 is aplan view of the nitride semiconductor light-emitting element 1′. InFIG. 11, region IA′ illustrates a cross-sectional structure viewed alongline IA′-IA′ in FIG. 12, and region IB′ illustrates a cross-sectionalstructure viewed along line IB′-IB′ in FIG. 12. This drawing differsfrom FIG. 1 in that there is a more dense population of large-sizedV-pits 15.

In Example 10, nitride semiconductor light-emitting elements focused oncharacteristics at room temperature and having the energy band diagramillustrated in FIG. 13 were produced. The following describesdifferences from Example 1.

(Preparation of a Substrate (Wafer) to an N-Type Contact Layer)

The preparation of a substrate and the configuration of buffer to n-typecontact layers are the same as in Example 1.

(Growth of a First N-Type Buffer Layer)

Same as in Example 1 except that the thickness was increased to 60 nm.

(Growth of a Multilayer Body to Growth of a Second N-Type Buffer Layer)

The multilayer body was formed as five stacks each composed of a 12-nmthick Si-doped InGaN (In composition, 0.04) layer and a 30-nm Si-dopedGaN layer. The rest of the multilayer body is the same as in Example 1.This structure omits the second n-type buffer layer 13.

(Growth of a Light-Emitting Layer)

In forming a light-emitting layer 14 on the top surface of themultilayer body by crystallographically growing well layers 14Walternately with barrier layers 14A, the well layers 14W were a total ofthirteen 3.0-nm thick undoped In_(x)Ga_(1-x)N (x=0.20) well layers. Thebarrier layers 14A, disposed alternately with the well layers, were atotal of twelve undoped Al_(y)Ga_(1-y)N (y=0.001) layers as thick as 12nm and were grown at a growth temperature higher than that for the welllayers by 140° C. The wavelength of the photoluminescence from thelight-emitting layer was 440 nm.

(Growth of a P-Side Intermediate Layer)

Same as in Example 1 except that the thickness of the p-sideintermediate layer 145 was increased to 10.0 nm.

(Growth of P-Side Nitride Semiconductor Layers)

Same as in Example 1 except that the thickness of the p-typeAl_(0.18)Ga_(0.82)N layer (p-type nitride semiconductor layer 16; p-typeimpurity concentration, 2×10¹⁹ cm⁻³), deposited on the top surface ofthe p-side intermediate layer 145, was changed to 32 nm and that thethickness of the p-type GaN layer (p-type nitride semiconductor layer17) was changed to 50 nm, p-type impurity concentration: 5×10¹⁹ cm⁻³.

(Etching and Formation of Electrodes)

No change from Example 1.

A resulting nitride semiconductor light-emitting element displayed bluelight emission with a dominant wavelength of 450 nm under operatingconditions of room temperature and 120 mA, with an optical power of 178mW and a voltage applied of 3.09 V. Although the optical powerpercentage of 80° C. to room temperature, or the thermalcharacteristics, decreased to 94%, this example was superior to Example4 in characteristics at room temperature. A comparative structurefabricated without the multilayer body had an optical power as low as155 mW, clearly demonstrating the effect of the multilayer body 120.

In this structure, doping the lower six barrier layers in thelight-emitting layer with silicon at 7×10¹⁷/cm³ reduced the voltageapplied to 3.07 V. The optical power was 178.5 mV, with no meaningfuldifference.

Example 11

In Example 11, nitride semiconductor light-emitting elements focused oncharacteristics at room temperature and having the energy band diagramillustrated in FIG. 14 were produced. The following describesdifferences from Example 4.

(Preparation of a Substrate (Wafer) to an N-Type Contact Layer)

The projections 3 a on the wafer top surface had a diameter ofapproximately 1.6 μm. The interval between adjacent projections 3 a(side length of the aforementioned triangles) based on apexes was 2.4μm, and the height of the projections 3 a was approximately 0.8 μm. Therest is the same as in Example 4. The configuration of buffer to n-typecontact layers is also the same as in Example 4.

(Growth of a First N-Type Buffer Layer)

Same as in Example 4 except that the thickness was increased to 60 nm.

(Growth of a Multilayer Body to Growth of a Second N-Type Buffer Layer)

The multilayer body was formed as four stacks each composed of a 12-nmthick Si-doped InGaN (In composition, 0.04) layer and a 30-nm Si-dopedGaN layer. The rest of the multilayer body and the second n-type bufferlayer are the same as in Example 4.

(Growth of a Light-Emitting Layer)

In forming a light-emitting layer 14 on the top surface of the secondn-type buffer layer by crystallographically growing well layers 14Walternately with barrier layers 14A, the well layers 14W were a total ofeight 3.0-nm thick undoped In_(x)Ga_(1-x)N (x=0.20) well layers. Thebarrier layers 14A, disposed alternately with the well layers, were atotal of seven undoped Al_(y)Ga_(1-y)N (y=0.001) layers as thick as 12nm and were grown at a growth temperature higher than that for the welllayers by 140° C. The wavelength of the photoluminescence from thelight-emitting layer was 440 nm.

(Growth of a P-Side Intermediate Layer)

Same as in Example 4 except that the thickness of the p-sideintermediate layer 145 was increased to 10.0 nm.

(Growth of P-Side Nitride Semiconductor Layers)

Same as in Example 4 except that the thickness of the p-typeAl_(0.18)Ga_(0.82)N layer (p-type nitride semiconductor layer 16; p-typeimpurity concentration, 2×10¹⁹ cm⁻³), deposited on the top surface ofthe p-side intermediate layer 145, was changed to 32 nm and that thethickness of the p-type GaN layer (p-type nitride semiconductor layer17) was changed to 50 nm, p-type impurity concentration: 5×10¹⁹ cm⁻³.

(Etching and Formation of Electrodes)

No change from Example 4.

A resulting nitride semiconductor light-emitting element displayed bluelight emission with a dominant wavelength of 450 nm under operatingconditions of room temperature and 120 mA, with an optical power of 180mW and a voltage applied of 3.07 V. Although the optical powerpercentage of 80° C. to room temperature, or the thermalcharacteristics, decreased to 94%, this example was superior to Example4 in characteristics at room temperature. A comparative structurefabricated without the multilayer body had an optical power as low as150 mW, clearly demonstrating the effect of the multilayer body 120.

Example 12

In Example 12, nitride semiconductor light-emitting elements focused oncharacteristics at room temperature and having the energy band diagramillustrated in FIG. 15 were produced. The following describesdifferences from Example 11.

(Preparation of a Substrate (Wafer) to Growth of a First N-Type BufferLayer)

Same as in Example 11.

(Growth of a Multilayer Body to Growth of a Second N-Type Buffer Layer)

The multilayer body was formed as seven stacks each composed of a 12-nmthick Si-doped InGaN (In composition, 0.04) layer and a 12-nm Si-dopedGaN layer. The rest of the multilayer body and the second n-type bufferlayer are the same as in Example 11. The number of times of stacking inthis example, increased from that in Example 11, was intended to keepthe same size of the V-pits as measured before the deposition of thelight-emitting layer. When the number of times of stacking was four, theV-pits were as small as approximately 120 nm across, and the opticalpower was reduced.

(Growth of a Light-Emitting Layer to Growth of a P-Side NitrideSemiconductor Layers)

Same as in Example 11.

(Etching and Formation of Electrodes)

No change from Example 11.

A resulting nitride semiconductor light-emitting element displayed bluelight emission with a dominant wavelength of 450 nm under operatingconditions of room temperature and 120 mA, with an optical power of 180mW and a voltage applied of 3.07 V. Although the optical powerpercentage of 80° C. to room temperature, or the thermalcharacteristics, decreased to 94%, this example was superior to Example4 in characteristics at room temperature. A comparative structurefabricated without the multilayer body had an optical power as low as150 mW, clearly demonstrating the effect of the multilayer body 120.

The embodiments and examples disclosed herein should be construed asbeing exemplary in all respects rather than being limiting. The scope ofthe present invention is defined not by the foregoing description but bythe claims and is intended to include equivalents to the scope of theclaims and all modifications that fall within the scope of the claims.

REFERENCE SIGNS LIST

-   -   1, 1′ Nitride semiconductor light-emitting element; 3 Substrate;        3 a Projection; 3 b Recess; 5 Buffer layer; 7 Underlying layer;        8 N-type contact layer (n-type nitride semiconductor layer); 10        First n-type buffer layer; 13 Second n-type buffer layer; 14        Light-emitting layer; 14A Barrier layer; 14W Well layer; 14WF        Final well layer; 14WI Initial well layer; 15 V-pit; 16, 17, 18        P-type nitride semiconductor layer; 21 N-side electrode; 23        Transparent electrode; 25 P-side electrode; 27 Transparent        protection film; 30 Mesa portion; 120 Multilayer body; 121 First        semiconductor layer; 122 Second semiconductor layer; 145 P-side        intermediate layer.

1. A nitride semiconductor light-emitting element comprising: an n-typenitride semiconductor layer; a light-emitting layer; a p-type nitridesemiconductor layer; and a multilayer body between the n-type nitridesemiconductor layer and the light-emitting layer, the multilayer bodyincluding at least one stack of a first semiconductor layer and a secondsemiconductor layer, wherein both of the first semiconductor layer andthe second semiconductor layer are doped with n-type impurity, thesecond semiconductor layer has a greater band-gap energy than the firstsemiconductor layer, each of the first semiconductor layer and thesecond semiconductor layer has a thickness of more than 10 nm and 30 nmor less, and each of the thickness of the first semiconductor layer andthe thickness of the second semiconductor layer is not less than athickness of any layer that constitutes the light-emitting layer.
 2. Anitride semiconductor light-emitting element comprising: an n-typenitride semiconductor layer; a light-emitting layer; a p-type nitridesemiconductor layer; and a multilayer body between the n-type nitridesemiconductor layer and the light-emitting layer, the multilayer bodyincluding at least one stack of a first semiconductor layer and a secondsemiconductor layer, wherein both of the first semiconductor layer andthe second semiconductor layer are doped with n-type impurity, thesecond semiconductor layer has a greater band-gap energy than the firstsemiconductor layer, the first semiconductor layer has a thickness ofmore than 10 nm and 30 nm or less, and the second semiconductor layerhas a thickness of more than 10 nm and 40 nm or less, and thelight-emitting layer includes a plurality of V-shaped recesses (V-pits)with sizes of 150 nm or more in a cross-sectional view.
 3. The nitridesemiconductor light-emitting element according to claim 2, wherein theV-shaped recesses (V-pits) reach the multilayer body at a bottom of aV-shape thereof.
 4. The nitride semiconductor light-emitting elementaccording to claim 2, wherein the V-shaped recesses (V-pits) are presentas a large number of scattered cavities in a plan view of a top portionof the light-emitting layer with a plane surface density of the V-shapedrecesses (V-pits) being 1×10⁸/cm² or more.
 5. The nitride semiconductorlight-emitting element according to claim 1, wherein the firstsemiconductor layer and the second semiconductor layer have equal n-typeimpurity concentrations.
 6. The nitride semiconductor light-emittingelement according to claim 1, wherein: an n-type buffer layer isprovided between the multilayer body and the light-emitting layer; andthe n-type buffer layer between the multilayer body and thelight-emitting layer is an Al_(x3)In_(y3)Ga_(1-x3-y3)N (0≤x3<1 and0≤y3<1) layer that contains an n-type impurity and is in contact withthe light-emitting layer.
 7. The nitride semiconductor light-emittingelement according to claim 1, wherein: an n-type buffer layer isprovided between the n-type nitride semiconductor layer and themultilayer body; and the n-type buffer layer between the n-type nitridesemiconductor layer and the multilayer body is anAl_(s4)In_(t4)Ga_(1-s4-t4)N (0≤s4<1 and 0≤t4<1) layer that contains ann-type impurity and is in contact with the multilayer body.
 8. Thenitride semiconductor light-emitting element according to claim 7,wherein the n-type buffer layer between the n-type nitride semiconductorlayer and the multilayer body has an n-type impurity concentration equalto at least one of an n-type impurity concentration of the firstsemiconductor layer and an n-type impurity concentration of the secondsemiconductor layer.
 9. The nitride semiconductor light-emitting elementaccording to claim 7, wherein the n-type buffer layer between the n-typenitride semiconductor layer and the multilayer body has an n-typeimpurity concentration less than an n-type impurity concentration of then-type nitride semiconductor layer.
 10. The nitride semiconductorlight-emitting element according to claim 2, wherein the firstsemiconductor layer and the second semiconductor layer have equal n-typeimpurity concentrations.
 11. The nitride semiconductor light-emittingelement according to claim 2, wherein: an n-type buffer layer isprovided between the multilayer body and the light-emitting layer; andthe n-type buffer layer between the multilayer body and thelight-emitting layer is an Al_(x3)In_(y3)Ga_(1-x3-y3)N (0≤x3<1 and0≤y3<1) layer that contains an n-type impurity and is in contact withthe light-emitting layer.
 12. The nitride semiconductor light-emittingelement according to claim 2, wherein: an n-type buffer layer isprovided between the n-type nitride semiconductor layer and themultilayer body; and the n-type buffer layer between the n-type nitridesemiconductor layer and the multilayer body is anAl_(s4)In_(t4)Ga_(1-s4-t4)N (0≤s4<1 and 0≤t4<1) layer that contains ann-type impurity and is in contact with the multilayer body.
 13. Thenitride semiconductor light-emitting element according to claim 12,wherein the n-type buffer layer between the n-type nitride semiconductorlayer and the multilayer body has an n-type impurity concentration equalto at least one of an n-type impurity concentration of the firstsemiconductor layer and an n-type impurity concentration of the secondsemiconductor layer.
 14. The nitride semiconductor light-emittingelement according to claim 12, wherein the n-type buffer layer betweenthe n-type nitride semiconductor layer and the multilayer body has ann-type impurity concentration less than an n-type impurity concentrationof the n-type nitride semiconductor layer.
 15. A nitride semiconductorlight-emitting element comprising: an n-type nitride semiconductorlayer; a light-emitting layer; a p-type nitride semiconductor layer; anda multilayer body between the n-type nitride semiconductor layer and thelight-emitting layer, the multilayer body including at least one stackof a first semiconductor layer and a second semiconductor layer, whereinan n-type impurity concentration of each of the first semiconductorlayer and the second semiconductor layer in the multilayer body is3×10¹⁸ cm⁻³ or more and less than 1.1×10¹⁹ cm⁻³, the secondsemiconductor layer has a greater band-gap energy than the firstsemiconductor layer, the first semiconductor layer has a thickness ofmore than 10 nm and 30 nm or less, and the second semiconductor layerhas a thickness of more than 10 nm and 40 nm or less, the thickness ofthe second semiconductor layer is not less than a thickness of any layerthat constitutes the light-emitting layer, and the light-emitting layerincludes a plurality of V-shaped recesses (V-pits) in a cross-sectionalview.
 16. The nitride semiconductor light-emitting element according toclaim 15, wherein the V-shaped recesses (V-pits) reach the multilayerbody at a bottom of a V-shape thereof.
 17. The nitride semiconductorlight-emitting element according to claim 15, wherein the V-shapedrecesses (V-pits) are present as a large number of scattered cavities ina plan view of a top portion of the light-emitting layer with a planesurface density of the V-shaped recesses (V-pits) being 1×10⁸/cm² ormore.
 18. The nitride semiconductor light-emitting element according toclaim 15, wherein the first semiconductor layer and the secondsemiconductor layer have equal n-type impurity concentrations.
 19. Thenitride semiconductor light-emitting element according to claim 15,wherein: an n-type buffer layer is provided between the multilayer bodyand the light-emitting layer; and the n-type buffer layer between themultilayer body and the light-emitting layer is anAl_(x3)In_(y3)Ga_(1-x3-y3)N (0≤x3<1 and 0≤y3<1) layer that contains ann-type impurity and is in contact with the light-emitting layer.
 20. Thenitride semiconductor light-emitting element according to claim 15,wherein: an n-type buffer layer is provided between the n-type nitridesemiconductor layer and the multilayer body; and the n-type buffer layerbetween the n-type nitride semiconductor layer and the multilayer bodyis an Al_(s4)In_(t4)Ga_(1-s4-t4)N (0≤s4<1 and 0≤t4<1) layer thatcontains an n-type impurity and is in contact with the multilayer body.21. The nitride semiconductor light-emitting element according to claim20, wherein the n-type buffer layer between the n-type nitridesemiconductor layer and the multilayer body has an n-type impurityconcentration equal to at least one of an n-type impurity concentrationof the first semiconductor layer and an n-type impurity concentration ofthe second semiconductor layer.
 22. A nitride semiconductorlight-emitting element comprising: an n-type nitride semiconductorlayer; a light-emitting layer; a p-type nitride semiconductor layer; anda multilayer body between the n-type nitride semiconductor layer and thelight-emitting layer, the multilayer body including three or more andseven or less of stacks of a first semiconductor layer and a secondsemiconductor layer, wherein an n-type impurity concentration of each ofthe first semiconductor layer and the second semiconductor layer in themultilayer body is 3×10¹⁸ cm⁻³ or more and less than 1.1×10¹⁹ cm⁻³, thesecond semiconductor layer has a greater band-gap energy than the firstsemiconductor layer, each of the first semiconductor layer and thesecond semiconductor layer has a thickness of more than 10 nm and 30 nmor less, and each of the thickness of the first semiconductor layer andthe thickness of the second semiconductor layer is not less than athickness of any layer that constitutes the light-emitting layer.
 23. Anitride semiconductor light-emitting element comprising: an n-typenitride semiconductor layer; a light-emitting layer; a p-type nitridesemiconductor layer; and a multilayer body between the n-type nitridesemiconductor layer and the light-emitting layer, the multilayer bodyincluding three or more and seven or less of stacks of a firstsemiconductor layer and a second semiconductor layer, an n-type impurityconcentration of each of the first semiconductor layer and the secondsemiconductor layer in the multilayer body is 3×10¹⁸ cm⁻³ or more andless than 1.1×10¹⁹ cm⁻³, the second semiconductor layer has a greaterband-gap energy than the first semiconductor layer, the firstsemiconductor layer has a thickness of more than 10 nm and 30 nm orless, and the second semiconductor layer has a thickness of more than 10nm and 40 nm or less, and the light-emitting layer includes a pluralityof V-shaped recesses (V-pits) with sizes of 150 nm or more in across-sectional view.
 24. The nitride semiconductor light-emittingelement according to claim 23, wherein the V-shaped recesses (V-pits)reach the multilayer body at a bottom of a V-shape thereof.
 25. Thenitride semiconductor light-emitting element according to claim 23,wherein the V-shaped recesses (V-pits) are present as a large number ofscattered cavities in a plan view of a top portion of the light-emittinglayer with a plane surface density of the V-shaped recesses (V-pits)being 1×10⁸/cm² or more.
 26. The nitride semiconductor light-emittingelement according to claim 22, wherein the first semiconductor layer andthe second semiconductor layer have equal n-type impurityconcentrations.
 27. The nitride semiconductor light-emitting elementaccording to claim 22, wherein: an n-type buffer layer is providedbetween the multilayer body and the light-emitting layer; and the n-typebuffer layer between the multilayer body and the light-emitting layer isan Al_(x3)In_(y3)Ga_(1-x3-y3)N (0≤x3<1 and 0≤y3<1) layer that containsan n-type impurity and is in contact with the light-emitting layer. 28.The nitride semiconductor light-emitting element according to claim 22,wherein: an n-type buffer layer is provided between the n-type nitridesemiconductor layer and the multilayer body; and the n-type buffer layerbetween the n-type nitride semiconductor layer and the multilayer bodyis an Al_(s4)In_(t4)Ga_(1-s4-t4)N (0≤s4<1 and 0≤t4<1) layer thatcontains an n-type impurity and is in contact with the multilayer body.29. The nitride semiconductor light-emitting element according to claim28, wherein the n-type buffer layer between the n-type nitridesemiconductor layer and the multilayer body has an n-type impurityconcentration equal to at least one of an n-type impurity concentrationof the first semiconductor layer and an n-type impurity concentration ofthe second semiconductor layer.
 30. The nitride semiconductorlight-emitting element according to claim 23, wherein the firstsemiconductor layer and the second semiconductor layer have equal n-typeimpurity concentrations.
 31. The nitride semiconductor light-emittingelement according to claim 23, wherein: an n-type buffer layer isprovided between the multilayer body and the light-emitting layer; andthe n-type buffer layer between the multilayer body and thelight-emitting layer is an Al_(x3)In_(y3)Ga_(1-x3-y3)N (0≤x3<1 and0≤y3<1) layer that contains an n-type impurity and is in contact withthe light-emitting layer.
 32. The nitride semiconductor light-emittingelement according to claim 23, wherein: an n-type buffer layer isprovided between the n-type nitride semiconductor layer and themultilayer body; and the n-type buffer layer between the n-type nitridesemiconductor layer and the multilayer body is anAl_(s4)In_(t4)Ga_(1-s4-t4)N (0≤s4<1 and 0≤t4<1) layer that contains ann-type impurity and is in contact with the multilayer body.
 33. Thenitride semiconductor light-emitting element according to claim 32,wherein the n-type buffer layer between the n-type nitride semiconductorlayer and the multilayer body has an n-type impurity concentration equalto at least one of an n-type impurity concentration of the firstsemiconductor layer and an n-type impurity concentration of the secondsemiconductor layer.
 34. A nitride semiconductor light-emitting elementcomprising: an n-type nitride semiconductor layer; a light-emittinglayer; a p-type nitride semiconductor layer; and a multilayer bodybetween the n-type nitride semiconductor layer and the light-emittinglayer, the multilayer body including at least one stack of a firstsemiconductor layer and a second semiconductor layer, wherein an n-typeimpurity concentration of each of the first semiconductor layer and thesecond semiconductor layer in the multilayer body is 3×10¹⁸ cm⁻³ or moreand less than 1.1×10¹⁹ cm⁻³, the first semiconductor layer containsIndium and the second semiconductor layer contains less Indium than thefirst semiconductor layer, each of the first semiconductor layer and thesecond semiconductor layer has a thickness of more than 10 nm and 30 nmor less, and each of the thickness of the first semiconductor layer andthe thickness of the second semiconductor layer is not less than athickness of any layer that constitutes the light-emitting layer.
 35. Anitride semiconductor light-emitting element comprising: an n-typenitride semiconductor layer; a light-emitting layer; a p-type nitridesemiconductor layer; and a multilayer body between the n-type nitridesemiconductor layer and the light-emitting layer, the multilayer bodyincluding at least one stack of a first semiconductor layer and a secondsemiconductor layer, wherein an n-type impurity concentration of each ofthe first semiconductor layer and the second semiconductor layer in themultilayer body is 3×10¹⁸ cm⁻³ or more and less than 1.1×10¹⁹ cm⁻³, thefirst semiconductor layer contains Indium and the second semiconductorlayer contains less Indium than the first semiconductor layer, the firstsemiconductor layer has a thickness of more than 10 nm and 30 nm orless, and the second semiconductor layer has a thickness of more than 10nm and 40 nm or less, and the light-emitting layer includes a pluralityof V-shaped recesses (V-pits) with sizes of 150 nm or more in across-sectional view.
 36. The nitride semiconductor light-emittingelement according to claim 35, wherein the V-shaped recesses (V-pits)reach the multilayer body at a bottom of a V-shape thereof.
 37. Thenitride semiconductor light-emitting element according to claim 35,wherein the V-shaped recesses (V-pits) are present as a large number ofscattered cavities in a plan view of a top portion of the light-emittinglayer with a plane surface density of the V-shaped recesses (V-pits)being 1×10⁸/cm² or more.
 38. The nitride semiconductor light-emittingelement according to claim 34, wherein the first semiconductor layer andthe second semiconductor layer have equal n-type impurityconcentrations.
 39. The nitride semiconductor light-emitting elementaccording to claim 34, wherein: an n-type buffer layer is providedbetween the multilayer body and the light-emitting layer; and the n-typebuffer layer between the multilayer body and the light-emitting layer isan Al_(x3)In_(y3)Ga_(1-x3-y3)N (0≤x3<1 and 0≤y3<1) layer that containsan n-type impurity and is in contact with the light-emitting layer. 40.The nitride semiconductor light-emitting element according to claim 34,wherein: an n-type buffer layer is provided between the n-type nitridesemiconductor layer and the multilayer body; and the n-type buffer layerbetween the n-type nitride semiconductor layer and the multilayer bodyis an Al_(s4)In_(t4)Ga_(1-s4-t4)N (0≤s4<1 and 0≤t4<1) layer thatcontains an n-type impurity and is in contact with the multilayer body.41. The nitride semiconductor light-emitting element according to claim40, wherein the n-type buffer layer between the n-type nitridesemiconductor layer and the multilayer body has an n-type impurityconcentration equal to at least one of an n-type impurity concentrationof the first semiconductor layer and an n-type impurity concentration ofthe second semiconductor layer.
 42. The nitride semiconductorlight-emitting element according to claim 35, wherein the firstsemiconductor layer and the second semiconductor layer have equal n-typeimpurity concentrations.
 43. The nitride semiconductor light-emittingelement according to claim 35, wherein: an n-type buffer layer isprovided between the multilayer body and the light-emitting layer; andthe n-type buffer layer between the multilayer body and thelight-emitting layer is an Al_(x3)In_(y3)Ga_(1-x3-y3)N (0≤x3<1 and0≤y3<1) layer that contains an n-type impurity and is in contact withthe light-emitting layer.
 44. The nitride semiconductor light-emittingelement according to claim 35, wherein: an n-type buffer layer isprovided between the n-type nitride semiconductor layer and themultilayer body; and the n-type buffer layer between the n-type nitridesemiconductor layer and the multilayer body is anAl_(s4)In_(t4)Ga_(1-s4-t4)N (0≤s4<1 and 0≤t4<1) layer that contains ann-type impurity and is in contact with the multilayer body.
 45. Thenitride semiconductor light-emitting element according to claim 44,wherein the n-type buffer layer between the n-type nitride semiconductorlayer and the multilayer body has an n-type impurity concentration equalto at least one of an n-type impurity concentration of the firstsemiconductor layer and an n-type impurity concentration of the secondsemiconductor layer.